I could ask the same thing about clans, and look how many of them there are out there today

Anyways, to the point. The purpose of levelgroups is for a group of people to work together on a large project that would normally be too much work for a single person to do. You also get the opportunity to work together with people who share the same interest (level/tileset making in this case) with the same amount of expertise which gives you the opportunity to see how they make stuff compared to you.

Personally I love the last one the most, I love seeing how people do things differently from myself. The greatest moment of my life was when MIRROWMAN came over to my house one weekend and we made Hotel Deluxe together, it was just plain awesome