If you share a file of 10Mb with another person:
In Dropbox that means 10Mb from your quota, and 10Mb from your firend’s quota
But in Copy, it will only represent 5Mb for you and 5Mb for your friend.
So, we can take advantage of this feature, lets say that you open one account for your PC, and another for your Phone, and you share all your files (put them all in one single root folder) from your PC to your Phone.
Take a look at this screenshot below, I have used 10Gb and shared them all with another account, so my quota is reduced only by 5Gb!