What's the point of NSAssert, actually?

I have to ask this, because: The only thing I recognize is, that if the assertion fails, the app crashes. Is that the reason why to use NSAssert? Or what else is the benefit of it? And is it right to put an NSAssert just above any assumption I make in code, like a function that should never receive a -1 as param but may a -0.9 or -1.1?