Basically they would stabilise the bike at speed, especially high speed corners. As the bike goes faster it becomes lighter and such is the way these bikes accelerate they constantly want to lift at the front. As you enter a high speed corner accelerating the front end has less grip so the wings would force the air down to give front end grip. If you watch slow motion shots of Stoner pre wing era he would slide the rear round so giving him a better chance of accelerating out the bends. The Ducati's were horrendous for front end grip and would 'fold' at the hint of a bend, even VR could not ride around the flaw. Because of this Ducati were the first to sport the winglets but other riders thought they were a safety issue as they stuck out like blades.