Lol, as opposed to all the other actors in the economy who provide a service.
>and the fact that they make the spread smaller is a bit of a side effect
It's not 'a bit of a side effect', it is what they have to do to earn fills or they will be undercut by their competition. Narrowing the spread is literally what a market maker is paid to do. It is their reason for existence. It isn't a 'side effect'. If you make a spread narrower than your competition, you steal his customers. Otherwise you don't trade and you don't make any money and you are out of business.
> Why call that a service?
It is a service. A very useful one. They offer, for a specified price, to sell (or buy) a financial instrument to (or from) you.
No one forces you to use it. If you don't want to, and think you can do better, you can take your chances and place a limit order in the book instead.
So like all services, it is completely optional, costs you money (to cross the spread), and provides you a benefit (immediate execution and certainty of price).
I suspect what you are struggling with, is that you don't realise that risk transferral is a benefit. Take for example, the dairy farmer who sells his milk to a company (a middleman) that transports it, pasteurizes it, bottles it and on-sells it to a supermarket chain. In this case, you would probably tell me that you can see what service they provide - they add value by pasteurizing and bottling the milk, which they specialize in. Which is true. But there is a hidden value too - they take risk away from the farmer. The milk could spoil, the trucks could breakdown, the vats be contaminated, the supermarkets cancel their orders. All of these are risks that the farmer would have to bear if the farmer wanted to bottle their own milk, take it to a market and sell it. Part of the service provided is to take risk away from the farmer and be paid for it.
That is all a market maker does. They offer you the opportunity to dispose of your risk (the risk that the financial instrument you hold may move against you in the time it takes you to find a buyer or seller) instantly by paying a very (very) small fee to do so.