It's an ego-driven medium. All of it: YouTube (when actually done for its intended purpose and not flagrant copyright flouting); facebook/myspace, foursquare, getglue. It's all about what you're doing and more recently _where_ you are when you're doing it. It's about the inherent need for self-fulfillment.
Let's say you have great taste in music. Just by someone noticing your daily playlist, you have an admirer. That admirer might want to know more about you based on that superficial quality, and follow you on your social update tool of choice. It's a tool of communication, much like a phone (actually more like a partyline, but I digress).
When it boils down to it, people need people, and as social creatures, we find every convenient avenue of acquiring and managing our social sphere as we can without expending too much energy. If we didn't care about any of that, hell, we wouldn't ever even comment in HN, right m0nty?