not all sci-fi conforms to starwars tech, gravity plating as you refer too is widely used becaused its easier. however gravity plating is advanced stuff, its far easier and less expensive to have instead a moving section to in part gravity. this ring could be just that. like i said, simple. The second possible could be thats its a shield generator? stay with me here; this race/empire could have a fleet of warships, but they have one problem, no shields. shield generators are huge and expensive with current tech-levels, so to prolong ship-life in combat a special support ship was made to help protect the fleet. this ship turned out to be the aptly named ring-ship, its sole purpose to provide shield cover for the fleet was they approach the enemy fleet, until they can unleash there own devastating broadsides. not every ship needs to have one if everything, every ship needs a role, a niche in which it alone can do.