You are still buying a copy, that I imagine is the practical effect you want.

You don't keep proof, though, and probably isn't allowed to keep a backup after you give the book away. But most countries laws don't care about any of this (and it's not a backup).

If you pirate then re-sell the book to a friend and they do the same, only one copy ever gets bought but everyone gets access forever.

Whereas of everyone buys new, keeps the book, and pirates, the author isn't going to see much negative impact.

Hanging on to the "proof" is important. Otherwise all you really prove is that you paid money to touch a physical copy.