Watching someone else play has some large advantages over playing yourself.
1) he has more skill at the game than you
2) he can put in the hours to get gear, friends to join his party, and other resources you don't have that let him do things you couldn't do
3) playing alone can be lonely. a streamer talking about the game provides some discussion about it. and streamers can be on audio chat with their friends, so you can be listening to a discussion between several people while they are playing. whereas if you play yourself maybe you don't have several friends at the same place in the game chatting with you about what you're doing
4) you can watch a stream on a computer that can't play the game
5) save money on buying the game (I often watch some gameplay footage of games i don't have to see if i'd like to play them or not)
So for example I watch http://www.twitch.tv/nl_kripp sometimes. He is in a WoW guild that I couldn't get accepted in, doing things in WoW I couldn't do. I want to see what high end WoW is like but I don't want to spend a year to catch up. So it's great to get a taste of gameplay that has a very high barrier to entry. I also save the $15/month for a WoW sub.
And for what it's worth, I bought guildwars 2 because of Kripp (for $60). It was fun to play the game while having his stream on. Why not do both in some cases!? I got tips from his stream and understood what he was doing better because I was doing similar things.