1)When something is so damn tasty, that it's not worth wasting the energy on describing it.
2) When savouring something so fantastic, and wanting to tell all, this is the easy way to get your message across.
Person 1- How's that steak buddy?
Person 2- (mouthful/mid-chew) NUM!
Person 1- Num..Num...
Person 2- That good hey?
Person 1- NUM!