I'm not very technical so pardon my language. Why does the client need the data and the key? Is client the same as end-user?

I understand that once a person decrypts a file, they could then copy and share that file, but I'm talking about a circumstance in which ONLY the intended person has the keys to decrypt the file. In that case, it wouldn't matter if the network still has a copy of the file, or if the originator decides to delete it from the network and some nodes refuse to comply because they'd just have an encrypted file in their node.