They've tried like every permutation of exp, checkmarks, badges, awards, etc over the years. Every time I open it up the design is wildly different. I guess they are optimizing for some lowest common denominator person (or rather, the kind of person who actually pays). But there's always levels with checkmarks, and skipping does give you all the checkmarks too afaik.
There was a design a while back where beating a level did NOT give you a checkmark. You had to "master" it by doing it 5 times, then you got the checkmark. That was truly horrible, I stopped using it immediately.
_foolmo_
he says listen to my story this maybe are last chance