Bragging rights as far as I can tell.
There are a few games where boosting helps (helped?) you gain higher tier items. In WoW TBC my team would sell spots on our teams. The buyer would stand around in a corner while my 2s/3s partner and I would play 2v3 win maybe 8-9 out of 10 games, and the buyer would then be rewarded with in-game currency used to buy items they wouldn't be able to get on their own. Eventually Blizzard patched the game and changed the rules so that it was not possible to boost without account sharing, so we stopped.
But, as far as I can tell that's usually an exceptional situation, and most of the time it's exactly what you describe - bragging rights that would quickly disappear if you tried to play on your own, because you'd tank your rating.