The ever-delightful Colson Whitehead wrote an essay for Publishers Weekly about why he's on Twitter:
I can only speak on my own use of Twitter, which is: I had a cat, the cat died, and now the stuff I used to say to the cat all day, I tweet.
That's my Twitter origin tale: it's nice to have a little company during the long workday. I'm probably more versed in why writers aren't on Twitter, as they constantly share their reasons without prompting.
It's a fun essay that includes a witty refutation of the commonly held belief that the internet is horrible for writers.