I think
@TwoSix is suggesting they don't know that ANY XP reward system can incentive play (correct me if I am wrong).
Sometime the play is the thing, as it were, and finding out what happens next is the primary driver. But I have played with many people for whom XP is the primary driver (often alongside or only slightly ahead of material rewards like magic items). Progression is baked into the system, and has been since the very inception of the game. How that progression is established has changed, of course.
Before finishing beating the dead horse, I just want to reiterate: I am against arbitrary GM XP rewards or milestone leveling because I think doing so has a negative effect on player agency. It usually goes hand in hand with the GM telling the players what they are doing next, which is generally not the kind of gaming I prefer (although, as I said, it can be fun sometimes to go on the rollercoaster e.g. Descent into Avernus).