No idea why they follow unknown people.
To talk to them? To read their tweets?
Every single person that follows me (apart from my sister lol) I never knew before-hand and before we randomly chatted, usually about sport and football. I now regularly go to Sydney FC games with quite a few of them, good lads too.
A lot of others that I talk to online a lot from different states, usually about football. It's a good social networking site. FB is better for interacting with your real life friends, Twitter is good for chatting with hundreds of people around the world who share the same passions as you.