As far as the role of the Gunboat, it's simply the fact I cannot really see the Empire being that 'big' on Fighter/Fighter battles. Why develop a long-range DogFighter when you can Hyperspace in a Battleship with 5 times more firepower than it could possibly need to achieve the objective?
That sounds like an Imperial-style approach.
However, something larger might be a threat to their big scary Battleship, or they might want to take out a supply convoy or the like without leaving their picket, it's roles like that which would need ships like the Gunboat, it'd be expected to be able to deal with some mobile resistance, but at the end of the day, it's never struck me as a Dogfighter. I know the Imperials do have some long-range dog-fighters, but I've always felt they were only developed near the end of the War, as an attempt to deal with the rapidly increasing fighter resources of the Rebels, particularly the B-Wing. The TIE Fighter could handle Y-Wings and Z-95's, but once X-Wings and A-Wings started escorting B-Wings on assault runs, T/F's were seriously out-classed.
Edit : Besides, can you imagine playing the entire TIE Fighter game in a ship without shields, when nearly everything you attacked did have shields, in fact, would you want to fly for a Navy that produces shuttles that are more intimidating in combat than the fighters? Which is probably the non-technobabble reason why they exist
