https://httpd.apache.org/docs/2.4/pr...apachectl.html

It is in the newest version of Apache, installed by default, and it works as I tested it on my hosting plan.

What is the purpose of it and is it something outdated and a backward compatibility thing? I think that all the things that it does can be achieved with systemctl too?

It is a "front end to the Apache server". "It is designed to help the administrator control the functioning of the Apache httpd daemon."

How does it work? There is no good info about it on the Internet.

The page you pointed at is literally the manual page for the command. So unless you clarify what you mean with "There is no good info about it on the Internet" that's just plain wrong. Also, unless / until you can 'sudo systemctl configtest httpd.service;' 'sudo apachectl configtest;' still has its role to play.

I meant that I couldnt find info about why it is needed, but I guess the commands that you mentioned have its use. So, like 80% of what it does can be achieved with systemctl, and 20% is still useful, type of thing. So that 80% of what it does could be probably considered outdated, probably... I dont want to sound like I am too particular about things, but I am going over a book and some other materials, and I try to figure out all the smaller things like this (second one so far, there is not a lot of this) that I dont understand.

Clear. Try to see it like this: 'systemctl' ships with systemd. That means it applies to Linux distribution versions that use systemd (so fore example Fedora-server, CentOS 7 but not CentOS 5 or 6). 'apachectl' OTOH is provided by the Apache foundation httpd package. It is a distro-agnostic command, and will work on any OS that exposes the command.

It is just a shell script so you can peruse the source with "less" and even if you do not program still get something out of it. One of the reasons there is not much about it on the net is that it is pretty simple.

As unSpawn mentions, it's there for cross-platform compatibility. apachectl will be present on the Linuxes, the BSDs (even OS X), and Solaris. So if you use it, it will be the same more or less everywhere. While the disto you are actually using might have something that supercedes it, that will not be portable. But it's not a big deal either way.