Boot partition:
Why have a separate boot partition ?

My reason is that I use LILO, and running LILO config saves a disk logical address for the LILO boot loader.
If the boot image should ever be moved, the LILO boot loader would not find it upon boot.
You would have to run LILO again to make updated boot data for the LILO boot loader. This is hard to do when your system won't boot.
Keeping all the kernel images in a Boot partition protects them against accidents such as moving one.

The separate boot partition can be left alone, and even if data the main partitions are moved, the Boot partition is undisturbed.
This makes for fewer opportunities for disturbing the LILO boot setup, and less chance of boot failure.

GRUB has one difference. GRUB keeps a full filename, so it is not bothered by the /boot image being moved. The GRUB loader has the ability
to read the filesystem, so the GRUB boot can find the image using the full filename (it does not use a saved disk address).

Also, when you decide to upgrade to a new Slackware, you will want to clean out your oldest Linux, to be replaced with the new Linux.
If that partition was where you were keeping the /boot images, they could get wiped out during the cleaning. They are safer in
a separate partition.

Would this only be for Windows 7 or newer versions upgraded online from a Win7 MBR installation ? Linux and BSD have no problem with GPT.

If you are re-partitioning the drive and restoring data later there is no reason not to switch to more modern GPT for any size of drive. You don't have any ancient 4 primary and extended and logical partitions where logical partitions must fit 'inside' an extended. All partitions (up to 128) are primary. You would have unlimited future modification capability.

The original poster has not given all the information on that system. Judging by disk size and what he is doing, it is close to my existing system, and I do run Slackware 14.2 instead of just reading about it. My original comments on using GPT are still the most relevant. The kind of reasoning used for defending why they choose GPT for their home system is not completely relevant to an older system or another user working on older hardware.
Linux, GRUB, LILO, and recent Windows will all understand GPT, but they are not the problem.
I do not trust that older operating systems like Dos5 will understand GPT. Unfortunately they will look at the partition table and will get upset if it is unreadable.
It will take a undetermined amount of time to research if those systems can be made to tolerate the GPT table.
The OP will know if they have DOS5 (or something else from before GPT), and can decide for themselves.
If you don't have the time, or patience, and don't have a terabyte-drive connected to it, then staying with the old DOS partition scheme will work for this hardware. I run one with 14 partitions and it has not given me any problems. FDISK likes cylinder boundaries, but I expect it would do that for GPT tables too.
I would prefer NOT to confuse a new Linux user with alot of demands that he has just gotta duplicate what someone else did, especially when it is not what he asked about, and when you cannot understand why he is doing things this way.