1: rewards players for simply showing up
2: provides players with a way to self-regulate the difficulty
3: makes players feel more invested in the characters, their save file, and therefore the game as a whole
4: adds new layers of complexity that aren't possible in standard action/FPS/other genres
5: makes (You) seethe
RPG mechanics are based and belong in everything because it's exactly how progress functions in real life, too. You do things, you gain those little % bonuses without even noticing it, and you become incrementally stronger until one day you look back at your previous self and understand how much better you've become through practice.