Why do you watch Let's Plays?
None of the above. And all of the above. It depends on the Let's Play.
Though I love to learn about new games I've never heard of, most of the Let's Plays I watch are because I like that Let's Player. There are a few that I like to rewatch over and over again because of that LPer's style (which usually ends up being the snarky, sarcastic, but not over-the-top ones).
Most of the LPs I watch are of games I've already played and beaten. Mostly because I either don't want to see spoilers on a game I want to play but haven't gotten around to it yet, or because I'm just not interested in the game at all, which is why I haven't played it.