The only reason I ask people to put down a reason for wanting to join our groups, is to know they are real people, and not a Spam bot. When we were actively using YG on a rare occasion a member's computer would get infected, and send out Spam to our members in our group. On those occasion I would tell the infected person on the side what was coming from their computer, and they needed to do something about it before sending any more posts to the group. If anymore Spam came from their computer, then they knew they were going to get kicked off the group. Once they got clean again, they could join the group again. I can't really remember a time when a person joined the group with the intentional purpose of Spamming the members or the group, so I think the practice of asking prospective members why they want to join a group is a good idea. I had to rely on YG to protect the group from bad things as much as they could, and I did what I could to help out the cause.
Don
Don Grass