Because writing is a solitary job, but writers are communicators, when we get together it's easy for us to become friends.
Of course the groups help us improve our craft and learn about professional opportunities. But, to be honest, I confess the main reason I participate in those groups is for the relationships. Even when we work in totally different genres and may not all agree with each others' points of view, we can still relate to each other and share one of the most important things in our lives.
In my opinion, anyone who is or wants to be a professional writer needs to get involved in at least one such group.