Good question - let me know if you find an answer. I has one two, but have no idea why. I keep getting e-mails saying "x is now following you on Twitter" and I think to myself "I hope they're not in a hurry to get anywhere, then".
Twitter integrates nicely with blogs (I have my five most recent "tweets" in my sidebar), so if you're in a situation where you need to report live-blog something in 140-character intervals and you're nowhere near a computer but you have cell phone reception, it's great.
It's pretty useless unless you have friends who also use it, or interesting people you just want to follow (and the latter can be difficult to find) just like Second Life is pretty useless unless you enjoy chatroom-style discourse or creating virtual things for other people to enjoy with their building and scripting systems*. Twitter's good for throwing out short ideas and bits of information and seeing where they land. Having IM windows open with dozens of people would drive me utterly insane, but twittering lets us talk about some of the things that interest us quickly and easily, while ignoring the stuff that doesn't apply and the snarking that's usually only witty in our deluded little minds.
*Or cybersex. But fortunately there isn't much of that on twitter.