As someone who has in fact completed both the original Gen 1 and the full Gen 2 Pokedex (including Mew and MissingNo.), I genuinely can’t imagine playing through a Pokemon game without at least completing the regional pokedex. Collecting the creatures is what I play those types of games for.

And the reward isn’t the little completion diploma Oak gives you to print out. It’s the self satisfaction that comes with finishing your goal. Like getting all the achievements in a game; I don’t get anything whatsoever for that, but I still like to do it. Because I’m a completionist.