this is a feature that works like this:

If the member made so much post as you entered by the group the member will become member automaticly member of the group. This Feature comes with 7.3.x

Yes, I know that it is a feature for groups that you have added but I don't remember seeing it for default groups before.

New registrations are by default added to the Users group so why would you need a Post Count option for it?

If you decided to have new users to be added to a different group, and then used the Users group for a special group.

I just worked on a site that created a New Users group with limited permissions, and that was where new users were added to. Then they were auto added to the Users group upon posting 25 posts.