Twitter is good for keeping people updated on cool things that are happening in your life such as traveling around the world, if you are famous or if you are trying to begin a following and what people to know your name.
I have a personal one but barely update it... I have a friend that updates hers every two minutes... it's annoying as hell... I don't care that her two year old daughter just spilled cheerios on the floor... really!! LOL I do like reading race updates, and desert related stuff.