"Boosting" is most common in team games, where some players believe they're being "held back" by their low-rank teams, and believe that having their account boosted will allow them to join the ranks of players that they belong in.
In practice, they're always wrong. All that a boosted account means is that the boosted player will ruin a bunch of games for other players until they drift back down to their true skill.
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.
example, recent highest difficulty full carries, where your character basically just dies at the start of each encounter so you don't screw up the mechanics, was going for more than ten million gold. With blizzard tokens returning about 110k/200k US/EU in gold that adds up unless the player has amassed that. There are end runs on the in game gold with very scripted in game conversations and money exchanged outside of game.
usually the reward is better gear but many do it for the achievements; these have become a cancer in my opinion in the gaming world; to special items like mounts and titles only obtainable to those who can do the content and do it in a timely manner.
We couldn't pull anyone past top 5% or so 2v3, so mounts were infeasible.
I totally believe that a lot of people would be doing that for real money though.