See screenshot.
The useless color band is showed on Google Chrome and Firefox.
Both old and new GOG designs have this band.

Waste of pixels, I have to say.

Oh, the useless band also appear at the bottom, but it is less annoying.

Because they like to see us suffer.

Because pixels are limited and we are slowly running out of them?

Because Barefoot essentials has eliminated it for some of us.

it's also on the very bottom of the page as well supposed to be for looks like a wall paper

I can honestly say that I have never noticed it until you pointed it out. Great ... now it's all I see :p

This is waaaay down the list of issues that needs fixings though.

What? Oh yeah,that's the second time I've noticed it and the first was in another thread where it was fleetingly mentioned.

It's called "style"! :D

Seeing how most modern monitors have way too few pixels vertically, that thing should go.

This!

It is a part of their plot to mind control us by subliminal messages and make us buy more games. By doing so, it will make us sleeper agents that GOG can trigger at any time as a part of an overlapping plot to eventually take over the world.

Or it is just a design decision.

Who knows?

TL;DR: Simply put, because they can...

If you are interested in a more detailed explanation with a little bit more background information (Warning! This will be a somewhat longer rant): Because they can.

You should be glad that you can see this thin purple line. It's a warning brought to you by a concerned web developer that the site will ultimately be engulfed and devoured by the purple ooze known as Galaxy. Repent, before it's too late!

I really thought you meant the galaxy banner...

Gotta be a weird design decision.

I always figured it was part of some aesthetic where the idea was those bits were intended to be the background with the rest of things overlaid on top of it. But the intention loses something in translation when all you see are smidgen-sized bars at the top and bottom of the page. If the stuff overlaid was even partially transparent, it'd at least justify the existence.