Is this an import from servers? Are they doing the same thing on retail now too?
The WoW token has brought it in for the most part. Since you can buy gold for RL cash legitimately on retail, you can then use that gold to buy boosts through content. People sell full clears of the latest raid for gold, as well as carries through almost all other content. The culture of it has then bled over into Classic as far as I can tell.