I don't know who thought it up, or even what it's for, but my best guess is that it's meant to be similar to the "badges" that you can earn for achievements in posting or the different colored stars that you get for larger quantities of buying and selling.  Probably meant to give us goals that we can work towards... though I don't know why anyone, including eBay should expend energy caring about such things... especially these days when the chat boards are just the merest whisper of a ghost of what they once were.