They probably don't add nearly as much value as they make for their players. However, they do add a small value: They close the economic arbitrage "loopholes" -

They probably don't add nearly as much value as they make for their players. However, they do add a small value: They close the economic arbitrage "loopholes" -- allowing other players to focus on other things, which are likely to have more economic value. They also may increase trust that the current pricing makes sense. When you buy some stock, it is more likely that you're getting a "fair" transaction.