Have you ever had an object in a xref that you wanted to copy to your drawing? Instead of opening the xref and copying an pasting the entities you want to move from the reference file to the drawing, you can use NCOPY and directly copy the objects without opening the file. The same applies for Blocks!
It works just like the Copy command, except it places the object on your current layer. It’s a great way to get linework from a referenced file or block into your current drawing.