INS is simply an internal system that feeds information to certain devices. INS is not a gauge or indicator. It can be translated if it was aligned properly to feed an instrument that indicates location. INS errors compound due to the fact that it is an internal system. After an hour it may be off by half a mile. After two hours it may be off by 5 miles.
So things like ILS, VOR, TACAN, ADF, NDB are separate. Those are external fixed systems that communicate via some form of EM. They don't lose accuracy.
INS would only apply for things like navigating to a way-point. If I want you to fly over a factory, I can't set up a radio beacon to guide you, and you can't rely on visual navigation. It'll most likely be pre-loaded via data cartridges (Mission editor) and you'l simply scroll through them and they'll show up on HUD or HDI.