So consider an insurance company. This company is engaging in transactions (providing insurance) in which one party will make $X and the other will lose an equal amount of dollars. Would you say that this company is providing no service?
It sounds like you are making the argument that this is zero-sum game, but whether something is zero-sum depends on your utility function. If the players are risk averse, then a transaction like buying insurance can yield positive utility for both participants.
Many trading strategies are performing a service in similar (but more complicated) ways.