Most people tend to think of TV watching as a passive activity - you sit there and watch and that's about it. Sometimes you can discuss it with other people after it's over, but it's not quite as fun and interactive as having an intelligent discussion while the story is unfolding.
Unfortunately, there's a downside to watching with other people. Talking about the show during the show tends to mean missing parts of the show. It's possible to rewind and rewatch, but that breaks up story momentum - plus it can be an art form to get it to start again at the right spot.
Fortunately, we now have text messaging. It makes no sound, and therefore drowns nothing out, and the receiver can choose when to read the incoming messages. Additionally it has scrollup - a written log of everything everyone said. This allows fuller, more complex discussions, especially after the show is over.
I prefer to watch TV with people over IMs or IRC. As someone who likes to pay full attention to a show and not miss a thing, watching with people in the same room tends to annoy me. Invariably they want to talk while it's on, if not to me then to each other, and they never wait for commercials. By having them over the computer instead, we can have all the fun of intelligent play-by-play discussions with none of the annoyance. Win-win for everyone!