To provide features that Xorg can’t.
If you don’t need features like fractional scaling, VRR, touchscreen gestures, etc. you won’t notice a difference.
People who do use those, will. Because for them, those features would be missing or not complete on Xorg.
In case of Gnome it was addressed, just by different people. Gnome 2 continues to live on as MATE, so anyone who doesn’t like Gnome 3 can use it instead.