There is an excellent description of why people use twitter in the Financial Times (FT) today, its called Sweet to Tweet.
I particularly like the Six Secrets of an Online Success:
Real-time flow of comments and adaptability to mobile handsets makes it more immediate than blogging.
Limiting messages to 140 characters makes them easier to produce and easier to digest.
‘Pull’, not ‘push’
The ability of users to choose whose tweets they follow makes it less random than e-mail.
Messages can be searched, making the content more accessible than the comments on a social network.
Mixing public and personal
A user’s personal contacts are on an equal footing with public figures.
By copying and retransmitting messages, users can turn the network into a giant echo chamber.
If you want a visual representation of the power of twitter, have a look at the fantastic people who follow my tweets and help me answer your UK property and construction questions(and other things of course).
Here are my Twitter Friends