Understanding the value of AP

I am trying to get a basic idea of what makes a unit good and at what (and got sparked to ask about it given a discussion about Howling Banshees vs MEQs). From what I can tell, AP is good when you have just enough to mess up the armor saves common to what you are trying to target. So like AP 1 or 2 is good against infantry because it makes their armor saves a lot more complicated. But it seems like alot of the things that higher ap would be good against also have either High toughness, or invulnerable saves. Which either represents a bigger problem, or caps how useful AP can get.

How much is AP "good"? Is it more or less important to have a good strength for what you are trying to get rid of?