Personal pride, I don't do them myself but a lot of my clan do so I play along with them and help as much I can, my brother has won 2 DOM capture ones and finished 3rd in last nights Core KC with 500odd confirms, in about half them 4-6 of my clan place top 20, they also see it as a way of putting us out there.

Another reason is to be headhunted I imagine, a lot of clans send my friends invites to join them after they do well so its possible people do them in order to find a better clan, couldn't do them any harm to have won a few of them when trying to join one.