I've been a member of the Writers of Chantilly for about 18 months, and it's made a tremendous difference to my writing. Before I joined, I thought the most important thing about being in a writers' group was the group's critique of the story. But now that I've been a member, I've come to realize the critiques aren't the most important thing, or even second. No, the critique is third on the list.
Second on the list is the moral support. Now, I've been writing since I was a little kid. Short stories, poems, screenplays, essays for school or even for myself. I guess I'm pretty well internally motivated. Even if I wasn't in a group, I'd still be writing. But I'm not sure I would have finished my most recent novel, or maybe it would have taken me much longer. The twice-monthly meetings of my writers' group energize me, supercharge me, make me eager to come back and write for the rest of the week.
But even that is subsidiary to what I've discovered is the primary advantage: being in a group makes me try harder. Where formerly I might have glossed over an awkward passage or half-assed a difficult scene, or skipped it entirely, I know now I'm going to end up reading that in front of other people, so I really have to polish my work. Make sure everything is exactly how it should be. Even if it's good, it gets an extra re-reading, and if it's bad, I keep going at it until I know it's something worth reading to the other members.
That's why for writers, I've come to believe being in a writers group is essential. It's something I would recommend to any writer. I'm not sure there's really another way, at least for me, and I suspect for others, to get out the best writing we're capable of.