it exists to keep the level of gold under control. 

in everquest, there was no way to make levels of platinum until way late in the game.  as a result, inflation got out of control.  items that sold for 200pp started selling for 2000pp because there was so much money floating around in circulation.  makes it impossible for new players to get started.

if blizz takes a cut of all the trades, that keeps our somewhat reasonable prices from getting out of hand.

video game economics ftw.