But why would that be important, given that Trump's people could've easily: ...
They could have, but they didn't.
So why go through the hassle of recovering the paper copies?
The paper copies are possibly evidence of a crime. The fact that the evidence could have been destroyed does not make it less useful as evidence. (Indeed, the fact that it wasn't destroyed makes it more useful as evidence.) Investigators gather evidence, so they recovered the paper copies.
The federal government also has a custodial interest in physically securing the documents to reduce the risk of the information in them becoming known to unauthorized people. The reaction to earlier requests to store the documents more securely led to the suspicion that the documents were at risk of being so exposed. This provides additional incentive to take custody of the documents.
Furthermore, the law specifies that any documents that fall within the scope of the Presidential Records Act are to be under the custodial control of the Archivist of the United States. The documents were seized in part to determine whether they are presidential records and, if so, to deliver them to the archivist. Whether Trump made copies is irrelevant to this, and, for documents that aren't classified and don't fall under the espionage act, it's irrelevant to anything. The presidential records act doesn't deny presidents access to their records. On the contrary, it guarantees access:
the Presidential records of a former President shall be available to such former President or the former President’s designated representative