I don't get it either. The nearest I can get is thats its a gossip facility where by you can voice an opinion on some subject that no-one gives a damn about, in the hope that some other pillock, also with nothing better to do, just may find some vague and remote interest in the subject to stir his brain to some form of activity.
I'm not into Twitter, but it obviously brings pleasure to a great deal of people. It also gives the opportunity for cynical, twisted and bitter Brits to sl4g off yet another thing in which they do not participate.
It's quite a useful marketing tool actually as well, as is facebook etc, you can reach literally millions of people once you have a following in a second and it's all free, I love it.