I guess I also don't "get" how it's a marketing tool. Facebook I get, MySpace I got. Other places I get. Twitter I don't get. Doesn't mean I won't do updates for Kritzerland or Nudie there, but I don't get it and I don't know that it does one bit of good. But if it makes others happy, then that's a beautiful thing. In a year or two it will be replaced by the next "thing" and in five years people will be scratching their collective heads as to why they ever thought Twittering was so novel or such a good idea.
It's micro-blogging. It's supposed to be small. It's meant to lead to other things, meant to link to other things.
As a record producer, you could post an announcement about your latest release - and post a link to the Kritzerland website so that anyone who comes across your "tweet" can purchase said latest release.
As a theatrical writer and producer, you could post the dates of your next production, and a link to where someone could make a reservation to attend the presentation.
There is quite a number of Sci-Fi enthusiasts on Twitter, and I'm wondering what would happen if you posted a tweet that included the words "The Creature Wasn't Nice"... You'd probably gain a new follower or two.
And as DR Druxy discovered, there's quite a network of writers and publishers who Tweet too.
Again, you don't have to get it. Just do it or don't do it.