The problem that designers face every day is that of unending revision. Perhaps something needs adding, removing, recolouring or de-emphasising. The permutations on how a piece of artwork can be subtly different are as infinite as the reasons for altering it.
Recreating physical artwork with minor differences is time-consuming and costly, whereas in the digital realm it can be quick, cheap and repeatable.
Using a graphics design application means colours and elements can be changed, removed or redesigned rapidly without a complete rework.