> a secure way for me to limit when and how you can use those
That would be having them access the file in a restricted environment (literally, guards and stuff). You can't have people accessing your secrets in the comforts of their own homes and at the same time not be able to reproduce them in some form – even if an exact duplicate of the original data would be infeasible to obtain.
It's the illusion of security.
The DRM contains the private key (because, well, you have to decrypt it, at some point). A motivated hacker will be able to get it, and decrypt the file for its own use.
If you don't have DRM backing it up, the scenario is "Here's a key, a locked box, do whatever you want with the decrypted file." Which is less secure than being able to actually limit usage of the decrypted file.
Nothing here implies that I think DRM is a good thing, mind you. I just think saying something is "just DRM", as if DRM wasn't basically encryption + limitations on use.