MMO operators have long struggled with secondary markets for virtual currency. They often have trouble trying to sell virtual currency directly to players because then it has the feel of "pay-to-win." And so third parties have captured the lion's share of this market for years -- by some accounts capturing billions of dollars annually in revenues.
But with WoW tokens (and similar concepts in other MMOs), they can market virtual currency to players, and because other players can feel like they benefit, too by getting their subscription for free, they don't complain as much of it being a "pay-to-win" model.
In reality, though, remember that Blizzard controls the market for both buying and selling WoW tokens, and they can peg prices (in cash and in in-game currency) at whatever they feel is appropriate, by either providing or reducing supply artificially.
Yet it is the artifice of "exchanging WoW tokens with other players" that gives the whole practice an air of fairness. This brain-dead-simple scheme will make them tens or hundreds of millions of dollars per year in increased profits that for years has been instead going to IGE and a cadre of third-party gold sellers.