To my knowledge, a regular Imperial Star Destroyer is a massive source of firepower and will intimidate most opposition in to not even trying to fight back. As for the few that would try and fight back, they can be stopped by fielding even more Imperial Star Destroyers.
With this, and the fact that Super Star Destroyers are obviously much more expensive to build and maintain than a good number of regular Imperial Star Destroyers, why did The Empire ever bother to build Super Star Destroyers? I can happily make an exception for the Eclipse-class - after all, it was The Emperor's flagship and had an amazing superweapon - but I can't understand anything like the Executor-class, which just appear to be really big ISDs.