Especially in older comics, some "supervillain" or "supervillains" (say, a gang with an armored flying car or a M-S* with an earthquake machine or an army of robots) hold a city hostage in the belief that once they achieve objective X no-one can wrest control of the city from them. Just what sort of schemes might a villain use to "Rule the City", apart from schemes where they run it through subterfuge?
Seriously, if he can take over a city but not the county/state/federal gummint, what's going to prevent the military from running in and smashing down the small-potatoes tyrant? After all, you don't want the rest of not-up-to-national-threat-level villainy to think you'll stand by and let some nobody take one of your towns without your permission? Besides, think of the lost revenue (taxes).
If you had a villain who wanted to take over HomeBaseBurg© and keep it, how would he do it? What are some good reasons why The Government™ wouldn't just send in the National Guard (or why they would be inneffective)? Why would supers be necessary to deal with situations military might should squash with ease?
* M-S = Mad Scientist, of course!