Because I trust some people's opinions, and if I am on the fence about buying a game, it's nice to look around. So Two Best Friends, Bro Team, Jim Fucking Sterling Son, Yahtzee, Mr. Tito and the highest rated Steam reviews all get a read or a watch.
In fact, Yahtzee is the person who most often sells me on games. If he says it's good, it usually deserves a bloody gold medal, and if he trashes it and I still want it, then I usually take that as a sign that it's worth playing. However, if he destroys the game, then I take his word for it because, well, he seems trustworthy and funny and as a Brit, being funny is about 9/10s of possession.