Service trademarks are often known simply as service marks. They are essentially the same thing as a trademark. The only difference is that a service mark is only used by firms that provide services (as opposed to those that make goods). A service mark, then, is a mark that identifies the source of services that are being provided. This could be something like the logo of a moving company that is painted on their moving vans.
There are two main functions that can be fulfilled by service marks. The first of these functions is identification. When a service mark is present, it tells us which company has provided a certain set of services. This allows us to be clear on the source of those services in case it is important to know that information. A second function has to do with publicity. Service marks are really forms of advertising. When a truck drives down the road with a given service mark on it, it is advertising to all that see it.
Thus, service marks are helpful to the general public because they let the public know the source of a given service and they are helpful to firms as publicity and advertising.