Any attempt to rationalize behavior like this assumes there is some sort of logical reason at play, the sort of thinking that rational people apply when they are weighing potential risks and benefits. With Trump, this is a futile exercise. He has never acknowledged that any law constrains him, or applies to him in any way. Considering his malignant narcissism, perhaps it just makes him feel as if he is still President to have this material in his possession. Perhaps he is just pissed off at being told he isn't allowed to have it. He is a child, an obnoxious spoiled brat who never grew up because he inherited enough wealth to escape any consequence of his pathological behavior. What I find to be even more mysterious than his malignant behavior, is that so many people still support and even worship him. Years from now psychology professors will make a career out of studying the contagious insanity that has gripped so much of the US, assuming of course the insanity hasn't sucked the whole world into Orwell's 1984.