What is a die?
A nibbler has a die, a punch and a punch guide that all work together.
The punch is the moving part that does as it says... it punches a hole out of the metal you are cutting.
The die sits at the bottom of the punch. The punch goes in to the die when cutting.
The punch guide guides the punch in a straight line whilst punching.
Why do you need a new die?
As with any moving part, things get worn over time.
The die will eventually wear out due to the metal chippings as well as the punch.