In the comments to a random post (it matters not where, only about what), I found myself writing a brief description of why I use Twitter which I thought was more or less superior to all my previous justifications for why I Twitter. I admit to not taking to the technology at first, but it has grown on me, so I thought I'd share. I recently realized I blog and post in so many different places that it seems pretty tedious sometimes to catch up, between blogging at both happyrobot and LiveJournal, Twitter, maintaining my Facebook page, manually updating GoodReads, Flickr, and Yelp, along with the automatic Last.fm updates everytime I listen to a song. Anyway, here's why I Twitter, despite all that other stuff.
Twitter, at least for me, fills a different hole in my life than email. Twitter might be for you if you change your status message on Facebook or Gmail Chat once or twice a day, as something funny flits into your head or across your desk. I tend to think of Twitter as a punchline delivery system, primarily.
Other people use it to let people know when they've posted something new (I tend to avoid people who only do that, as that is why I have RSS feeds, but it can be useful when it's not redundant).
Hopefully, most people don't use it for what Twitter claims to be: updating people on where you are. I could really care less if you're in the line at the post office to buy stamps and you're bored. But if you use it for very brief funny or insightful things as they come to you (via text message or IM or the like), certain people's Twitter accounts can add to your day.
Some of the defaults--which can result in everyone's Tweets popping up in your email or as text messages--can be irritating, but if you can find just the proper level of obtrusiveness so you're getting and reading the messages regularly without them distracting and irritating you, it can be a real benefit. TwitterFox for Firefox, the TwitterGadget for Google, and Twitterrific for iPhone are the methods I use, but some people just go to the main website on a semi-regular basis or add something in Facebook if they use that every day.
Not everyone is good at Twitter, but that's true about pretty much everything. I hope that I'm good enough to keep people interested, at least. And I find it interesting enough to keep at it, for now. I follow about four dozen people, and about four dozen people follow me, so it works out.