They help me work out my plots, get to know my characters, and rehearse their dialogue. In fact there have been various occasions when I've realised from the reactions of passers-by that I've been speaking my characters' parts aloud in the street. It's more than embarrassing when it happens, and one of the great delights of talking to other writers is to discover that I am not alone in that particular eccentricity. Most writers are a little odd. It comes, I think, from spending so much time alone with one's imagination.
Research is often more fun than writing, as is promoting the latest book.
Both involve talking to other people, watching their reactions, getting the kind of instant feedback that publishing rarely provides. Sitting alone at home with only the word processor for company, wondering whether the novel you are writing is going to work, can be a lonely business. But it's a loneliness that is important. For me, characters are the most important part of any novel in any genre. Of course the style matters, and the plot, but if I am not interested in the people about whom I'm reading, then I don't want to bother.