A copy can pave the way for realising capital on an original painting, or for making a gift to a relative.
It takes away the constant headache of security and insurance premiums on a valuable original.
It is one way to make a themed room of, say, sporting pictures, where originals would be prohibitively expensive.
If one member of a family inherits a painting, a copy is a solution for the others who have not inherited.
It is one method of acquiring a painting at a reasonable price where the original would never come onto the market or would be hugely expensive.