I mostly watch them for the same reasons I listen to most podcasts: it's people talking in the background while I'm doing chores or whatever. I only watch videos with at least two people because otherwise it's intolerable even if I really like the person. In a lot of cases I'm only looking at the screen maybe a quarter of the time.

I'll also watch videos to decide if I want to buy a game, to learn about a game I won't play but seems important, to experience the story while doing other things, or to learn how to be better at the game.