You're not quite far enough from the thick of it. Your weapon is very, very powerful, but invariably not quite powerful enough. You can easily be attacked from the rear and sides as well as from the front, and you'd better not have your eye to the sights when it happens. The Sniper's basic job is to snipe. To keep up a suppressive fire that forces the enemy to reconsider standing around outside observing your base at leisure.
Snipers are also useful at flag defense.
You can keep your red dot trained right on the flag and time your trigger release to the invader's flag-grab. Distance is indicated. The Sage of Infinite Wisdom will cross the Great River if he values his hide. Snipers can go offense, switching to number 3 or 4, which gives you full-auto on the Sniper Rifle or the nailgun, both of which are potent weapons in the right hands. Snipers can give important information to team-members, particularly the Engineer, who sits downstairs, blind except for your data.