The most important role of the spotter is security. Not always, but teams don't need to be overburdened with extra equipment meaning the spotter is toting the firepower. In the event of unexpected contact the spotter will lay down mass fire while the sniper finds a position to cover the spotters retreat. The second most important job is in adjusting fire for the sniper in the event it is needed. At one time the spotter was responsible for carrying communication equipment but in airsoft, both the sniper and the spotter carry comm's.
Spotters have binoculars and spotting scopes, those items provide a greater field of view than does a telescopic sight and are much less likely to attract attention than swinging around a sniper rifleās 20-inch barrel. The spotter (or observer) assists in site selection, camouflaging, target selection and observation, intelligence gathering and reporting, range estimation, as well as observing things outside what the sniper can see in his scope.
Also consider that the intelligence provided by sniper/marksmen units are used far more often and by far more people than their actual shooting skills. Information gathered is relayed to your teams squad leader. This way he can formulate the best course of action to complete the objectives.