thank you for your super helpful response, (going to test this in a mo!)
I have been given more some thought to your question before you asked it, I am still learning unity's animation as I have come from UDK, but the main reason would be for example, reverse animation, there may be several steps to a final menu with different animations at each step, and if you wanted to reverse this animation, instead of making another animation combining all of them, you would simply use the same animations for going forwards in a reverse linked chain.