I'd say mostly for the middle ground. Celebrities and politicians (of any significance), have already "made it" and they don't really need Twitter aid their reach - mainstream media is all too ready to cover their stories. At the "low" end of the spectrum you have people who are basically consumers - they might not produce much interesting content but they are important for content creators. In the sort of middle ground you have people who are making stuff - youtube videos, games, applications, art, crafts, books, comics -- even just meaningful Twitter commentary. Twitter makes it easier to find and interact with these people than alternate solutions (Googling, Facebook, etc). I'm not sure how much value Twitter really has for generic businesses - I think its more about individuals than the businesses they run (this can easily be seen in the amount of people who follow company accounts vs individuals).