I think there are a number of complex psychological factors here. I'll break them down.
It's fun. Dozens of other teachers I have never met are all talking about a subject of interest to me.
It's ego boosting. Often I say something in a chat that other people like. I get the immediate gratification of seeing them respond, retweet, favorite and in may other ways approve of what I tweeted.
It's educational. Many of the questions will push my thinking. Other participants will share resources that I will dig into further.
It's inspiring. I love seeing what other teachers are trying or watching them get excited about something they want to try with their students.