Well, there are few reasons for GM:
- Better use of screen real estate
- Design (without it we get empty, often non-symmetric spaces that serve no purpose and MIGHT offend someone’s taste - moa .
- Design again, windows get cleaner and minimal look.
- GM works best with upper panel which is typical for Linux setups, so we get non-windows design and feeling, while non GM setups work best with windows alike layout, which doesn’t create this unused stripe next to unused stripe next to another badly unused stripe which happens with upper panel. However, there is a setup that has windows alike look, but there is an invisible upper panel that becomes menu panel when maximized so you save space for titlebar.
- GM invokes a feel of integration of apps with the desktop and you know they are native and cooperating apps, not any wine-wrapped monstrosities that won’t have GM.
But of course this is above all a personal taste. I can understand dislike this ever changing, confusing, awkward element. GMs can feel like that so if you don’t get the pros like above, all you are left are those mixed emotions of understanding that it’s good for real estate but you absolutely don’t like how it looks and works.
Luckily Plasma gives us freedom to chose whenever layout we prefer. I hate when we have no choice (Unity - forced GM, Gnome - force no GM). Linux is about freedom so currently Gnome feels not fully like Linux but more like a proprietary systems like Android.