You are under arrest, my lord.
-Mace Windu, Revenge of the Sith
I never broke the law! I AM THE LAW!
Hannah Arendt, who wrote the classic analysis of totalitarianism,said that totalitarian societies had three characteristic institutions:massive propaganda efforts directed at their own populations, secretpolice forces, and concentration camps that caused mass deaths.
-Bill Stoddard, Social EngineeringA police force imposes the will of the government on the people. The Imperial Ministry may make the law but Imperial Security enforces those laws. More than that, dictatorships need police, taken to extreme levels. The Empire is almost certainly CR 6, and it must therefore police its people to make sure that no "sedition" or "corruption" has seeped in, and that nobody threatens to topped the delicate structure upon which the Emperor has settled his throne. All must embrace the cause, all must understand the divine purpose of the Emperor, and all who do not must be found, convicted and shipped off to labor camps to die out of the public eye.
More than any single force, I expect the players will run up against Imperial Security. When they try to assassinate officials, or smuggle medical supplies onto a rebellious world, or when they're trying to make sure their machinations aren't uncovered by the empire, it's Imperial Security, not the Imperial Military that they'll face. Thus, we need them, both as someone to serve (especially if you're a Bounty Hunter), but especially as opposition.