1. Avoiding DRM hell. I have security DONGLES and paying for software license that then do you no good when you need the software. FOSS and PD software always seemed smarter to me.

2. Speaking of paying: return on investment. I do not mind paying for shareware once I am sure it fits my needs, but unless you are MAKING big bucks with your software paying big bucks FOR your software is just stupid!

3. Security. This one is not intuitive, but often the FOSS software is better secured FOR me than commercial software. The focus on commercial software seems to be securing it FROM me.

4. Quality. Commercial software often has a more polished user interface, but less correct back end. I can put up with a slightly rough user interface to avoid data corruption or improper behavior. That back-end quality is more important than look and feel.

Microsoft is out to make money for share holders - sure. And rich companies are annoying - but I have to say they have definitely stepped up their open source game.

http://open.microsoft.com/
https://opensource.microsoft.com/
https://github.com/Microsoft
https://mva.microsoft.com/

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I use open source software because there is a lot of good open source software available and it's generally free. Though throughout my history of using open source software it's typically been for my own purposes, a lot of open source software has advanced to the point where my clients can see the benefits of using open source. I see a time in the not too distant future where I can develop value added solutions for my customers using open source software.

Instead of "open source," let alone "FOSS," I like to use the term, collaborative development.

Many people throughout the world can successfully cooperate on the building and maintenance of a piece of software when it is both understood and legally-enforceable that (a) no one can claim proprietary rights to the work product, and (b) everyone must share. Agreements such as GPL were crafted by legal professionals and have been affirmed by courts and governments around the world.

The result is a rising tide that lifts all boats. The result is also generally a piece of software that is better than any one developer or company could have done – or, perhaps, could have afforded to do – on its own. Software development is frightfully expensive. Not every contributor faces the same business requirements or technical challenges. However, if every contributor conquers their specific need and then shares the result, the software gains the ability, and no one has to pay for it. This enables collaboratively-developed software to "grow by leaps and bounds," in a way that could not have been economically justified by the conventional for-profit business model.

Many very successful proprietary systems have been built on top of open-source foundation layers, and this is expressly permitted. Many vendors of open-source software also sell proprietary additions that are only available for purchase. This, too, is permitted.

It is also worth noting that all of the major software systems – including Linux itself – are supported by foundations that are backed by very major players in the software industry. And there's a reason for that.

I've never had the open source community tell me that what I am seeing on my system can't possibly happen. Generally, the weirder the problem, the more people are interested in figuring it out.

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Mainly because its free to use, also it has freed me from the scourge of viruses and also because of the great help and friendship that is found in the various forums and chat rooms.

but the question was about the software.

developers of FOSS are (usually) not interested in hiding flaws.
if they are, it is soon discovered and discussed (and i try not to use software that doesn't leave this option even if it's foss).

that is what i meant by "no-one is trying to sell me anything".

but i grant you this, it doesn't hold true for 100% of all F(L)OSS apps out there.
but at least i can make an informed choice, which usually cannot be said for $PRORIETARY_OS.

It works, some times a lot better than commercial software.
It doesn't tie you in.
You don't have to constantly upgrade to keep on using it.
It's free to use, how you want to.
It's free to modify, if you have the skills to.
It has a great community behind it.
You can become a part of the community.
Just some of the reasons why I use it.

I use Linux for several reasons: I don’t like being pestered by Microsoft flogging their latest version of Word etc. I have a dual boot because sometimes I have to use the windows 10 that my machine came with. I progam in Fortran and the free Gfortran in Linux is excellent. At the moment I feel Linux is more secure. Alas I am not a true nerd but when I execute a single command line instruction or tell someone I use Linux I get a pleasant invert snob satisfaction.
That all from royden

I use Open Source software because I don't have to worry about it going obsolete when a company goes out of business or decides to stop supporting it. I can continue to update and maintain the software myself. I can also customize it if the software does almost everything I want, but it would be nice to have a few more features. I also like Open Source because I can share my favorite programs with friend and coworkers.

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Open Source: Good to very good and (through repositories for Linux) always avaliable.
For MS-Windows the site sourceforge.net that site was failing in reliability. I do not the status of that site now. The Linux repositories are easy to use and you don't have to go to the shop.

If an application is not doing what you were searching for, continue searching in stead of trying to get youre money back.

Added: march 4-th, 2017
At the moment I'm in thailand, surviving the dutch winter, and in Thailand I can buy computers withouh MS-windows without a expensive Microsoft license. Here you have to pay an extra 4000฿ for a microsoft license. Try that in Europe!

Rather obvious. The answer is partly already given in the question:

...OPEN SOURCE SOFTWARE...

1) It's open, free, accessible to anyone.
2) For those capable of: open software can freely be changed, adapted, improved and shared.
3) The feeling that if the download and use are free, one can always freely send some financial or collaborative compensation to the community: that curious feeling of having the choice whether to pay for it or not ;o)

The answer to this question is easy: there is NOTHING that I use that can't be found in Open Source programs. Regardless of what Windows or Mac offers, I have found an Open Source equivalent that is either equal to or better than the 'proprietary' program.

Of course this may not apply to many people but, for me, Open Source is just about all that I need.

I must say that I do use some 'closed-source' codecs in order, for example, to play MP3 files.

But that's about it. I am full of admiration for those who design and make available FREE OF CHARGE the Open Source programs and, for these programs, I sincerely thank the program designers.

When I started to travel the computer road, back in 1977, I ended up buying one after the other. At that time, the price of the hardware was so high that the cost of software was next to irrelevant. For example, I remember buying a 10MB hard drive for over 1,000$ or 256K of RAM for over 500$ back in 1985! Therefore, an 80$ spent for an MSDOS was considered fair when a PC would cost in excess of 3,000$.

Now a day, A PC may cost as little as 400$ and does 100 times more work than in 1985.

Software wise, the opposite has been happening... Software cost 100 times more and require 100 times more hardware to do some 10 times more than 35 years ago.

So, why did I go the Open Source path?
1- My computer hobby cost me less than half of what it used to, in part because I switched to Open Source.
2- I can give support to any person or group I choose, based on the quality of their contribution.
3- There is little concern for viruses, Trojans, worms and stuff of that kind.
4- When I update the software in my PC, ALL of it is updated. Micro$oft still can't do that!
5- My PC almost never bomb, being left ON 24/7 for months at a time. (my personal record is 487 days)
6- I can read and write almost any file format or file system in existence. (Micro$oft can't)
7- I can do, under Open Source, almost everything that is available under proprietary ruling.

Keep in mind that computing is a very nice hobby I intend to keep active into... Especially for a retired guy!

I'm a huge fan of open source for several reasons:

1.) You can see what is being done in the code. This aspect alone would be reason enough because it allows not only the chance to learn how to code better, but to verify that the code is not doing something nasty. Even if I'm not personally sharp enough to find such flaws, you can bet many others are - someone WILL figure it out and post it, and earn some well deserved major bragging rights for doing so.
2.) If you are so inclined you can contribute changes (bug fixes, upgrades, whatever) and not only improve the code, but, build skill and experience too. Its great to show your employer (or future employer) how you can hack some serious code.
3.) Being a part of something awesome, something owned by the collective community and useful to many people that appreciate your efforts. Like me, I owe a huge debt of gratitude to the countless-many super-smart and kind people that contribute their code - THANK YOU!
4.) Maybe one day giving something back that others will benefit from. The idea of serving a great community appeals to me very much.