What is the purpose of a wheel bag?

Why the complaint? It cost you nothing...

And, I use my wheel bags when I put my bike in the trunk of the car. Frame in first, then wheels on top. Protects the wheels, car interior and bike frame.

not really complaining just a big bulky bag.  I don't have the type of rack that requires a wheel to be taken off.  Maybe I'll just ebay it.

Some will bring a second set of wheels to races as well. Criterium bike racing and cyclocross uses "pit" wheels. You get back to the pit area and change the whole wheel for a mechanical.

Also, for time trials and tri, some will bring standard spoked and aero wheels and choose depending on what the wind is like.

Personally, I like the wheel bags.

I can see why someone would like it if they had a rack that required wheel removal.  I've seen wheel bags and those are nothing compared to this big thing.  Also makes sense about bringing different wheels. I race mountain bikes primarily and I will change tires as needed.  

Until you start travelling with you bike in your car, you probably won't appreciate the wheel bags... if dislike them that much sell them to someone that do appreciate them.