content: They fire projectiles without relying on explosive combustion that can in theory reach the destructive potential of missiles (and beyond.)  The no combustion part means the ammunition needn't have explosives packed to it and is going to be more stable, less prone to degradation, take up less volume, can be molded with little concern other than kinetic potential, and finally it's probably also cheaper on top of all of this. If past technological evolution repeats itself here in a decade or two we'd have ships capable of blasting off dozens of rounds per minute of projectiles with the same destructive potential as missiles.