From what I understand of warfare in Starwars, Its all about orbital controll, the big ships have essentially won a planet if there is no opposing fleet and the planet has no shields. Each big ship is a KillSat that will basically do this to anything on the planet it doesn't like...



But if a planet has shield generators, than troops have to be deployed on the planet itself to take the generator out. And the small ships can help in that, that is their use, they can fly low in the orbit and assist the ground troops.

While in space battles they are pretty much only annoyances, but fighters and bombers can assist in taking out certain targets. Bombers carry bombs (of coarse) that can hit hard, and fighters assist the bombers with their own torpedoes and protect the bombers from enemy fighters, as well as taking out enemy bombers and their fighter escorts.