X11 is a super old system from the 80s designed for computers the size of our living room.
Wayland is pretty new and made for today’s devices. It’s simpler code wise (therefore easier to maintain and contribute), ensures modern functions (like smooth 1:1 gestures), is more secure, smoother, draws less power/ ressources, could enable HDR in the future, better multi monitor support, and much more.
It’s a difference between night and day. If you use Gnome or KDE, try it for some time. Wayland is usually the default for both by now and X11 only exists for legacy stuff.