1. It's where the people are. I don't think anyone is 100% loyal to it, but you have to respect its reach.
2. Instant answers. I can get insight on news via Twitter well before I can get it anywhere else. On top of that, I can get it from the people I want, many of whom are Tweeting hours before they are able to get an article published. This is great for sports, politics, etc.
I personally don't use Twitter nearly as often as I once did, and I rarely find reason to Tweet, but I do enjoy browsing it from time to time.