Right, I understand your confusion, because I was confused at first, too. Groups make it easier for people who enjoy only a certain type of system to communicate with each other easier.
Although people can post to the Apple II forum, we can share pictures, resources, etc. here much easier than through the Link Exchange area, which most people tend not to look at. Nothing secret, just makes it easier.
Also, we can discuss certain topics here without crowding the Apple II forum, and we get a cool "Apple II Users" badge in our profile pages. :D