Why did Trump want all those documents in the first place? Either it's because he wanted to have them as some sort of mementos or trophies, which would be stupidly childish or he wanted to have them as assets, which would be brazenly callous towards his own country even by his own standards.
My personal theory is just that he was convinced he'd be made president again in a matter of months, if not weeks, and didn't want Biden to have them in the meantime.I certainly lack the imagination to come up with a theory like this, but it does make a bizarre sort of sense, because in Trump's delusional mind, it would make him a savior that protected the country from Joe Biden.