CR in 2014 game explicit benchmarks for things like HP and damage output. it was not perfect but at least it was something. LevelUp examined that and tried to make it more useful and explicit. Out of that the real useful elements popped out: the top CR reasonable for a specific APL. If you were to get rid of CR and just go by XP value, you would end up with GMs choosing way too powerful solos a lot of the time.