Writers argue about a lot of silly things, like whether writing groups are helpful. The sane answer to that? Writing groups are like any other group: good groups help some people, but some people are better off solo.
Five Temptations That Actually Boost Your Willpower at Psychology Today has one reason why good writing groups help some of us: "Research shows that willpower can be contagious. You can “catch” extra self-control just by seeing someone else pursue a goal."