I'm a known addict of Twitter. Maybe I should try to sound professional and say my reasons for using it are related to work or writing, but I'd be twisting the truth.
I first went on Twitter because it was mentioned in a writers' email loop and it sounded as a good way to meet other writers, both published and not published. I don't know any English ones in Spain and the long journey to publication can get pretty lonely. My first aim was met in only a few days and I have been meeting people related to the writing and book world ever since. You get advice, courage, and understanding from other writers and authors who've suffered the same frustrations and fears. In fact, I've been pleasantly surprised by how approachable many of the successful authors are. They are more than willing to share their experiences and advice.
I've also met people who aren't related to the writing world but I have still learnt from them or they've simply made me laugh. Somehow you tend to find people you have something in common with and this makes you feel part of a friendly community.
However, I have seen people stalk and pester other people to achieve their career objectives and it makes me feel anxious for their victims. For instance, Carole Blake, a well-known literary agent, often has to remind her followers that you do not pitch your books on Twitter unless you're ready to risk being blocked if you do. On the other hand, there are job opportunities announced on Twitter, but the advertisers are asking to be "pitched" or will indicate an email to do it correctly, and I suggest you do follow their guidelines.
But all in all, my real reason to visit Twitter day after day is the fun and the need to cry out to someone when I'm alone at my desk. Working at home has many advantages but there are times when you really miss having someone to talk to, even if it's only to complain about the insurance sellers who call day after day to drill into your head all the catastrophes that are looming ahead.