Almost all exchange tokens are just a way for exchanges to make more profit. They try to promote a token by giving it's users discounts for holding it while they actually hold the majority of tokens in their own wallets. After the token has gotten some traction they can start pumping it more and slowly selling their own bags while the users are buying them all because they get a small discount on fees. Inherently the tokens have no use so IMO there is no reason for them to exist except to make money for the exchange owners.
You just stated the obvious that exchange token are there to have discounts to their service. Though, it is true that creating their own token is one way of an exchange to crowdfund their exchanges and bring more quality service to their client but it is better than those ICO/IEO that offers vaporware.