What exactly is DRM supposed to achieve? For it to work it seems to need to prevent 100% of all opportunities, worldwide, of duplicating copyrighted material. A

What exactly is DRM supposed to achieve? For it to work it seems to need to prevent 100% of all opportunities, worldwide, of duplicating copyrighted material. A single copy is all it takes to seed every single pirated copy. I can't see that the sales of DVDs and Blu-rays are going to dry up any time soon and given how easy it is to copy those how does DRM help at all?