For me I like to see that there's actually a person behind the screen. When I watch a stream without a webcam I become bored quite quickly as I feel there's no interaction between the viewers and the streamer and I can't see their reactions or tell what they might be thinking.
This. I think it's just more entertaining to have the person playing have a webcam because there's more interaction. For example, when you ask a question in chat or say something and the person streaming alt+tabs to chat and you can tell he's reading it and then responds to the question, it's like he's talking to you instead of just a random voice answering the question. This is why people like C0mplex1ty always have 100s of viewers even though their playing itself isn't "pro" level. (Also, boobs.)