There‘s a ton of speculation from political types as to what Trump would have done with the nuclear documents (he also apparently had intel about how the CIA spies on people using tech, too).
1. The most popular rumor is that Trump might have intended to use the materials as part of a $2 billion loan Jared Kushner wanted to secure from the Saudis. (I don’t know, $2 billion seems like a bargain for that kind of intelligence, honestly.) This has been speculated to be a thing as early as 2019.
2. Some think Trump may have been keeping the materials as a way to get asylum in a country that wouldn’t extradite him back to the US (Saudis, again). He would have to trade intelligence for asylum since his fat ass is flat broke.
3. Another rumor is that he kept the stuff to show off. Dumb, but he is dumb.
From what I heard, Trump was keeping all of these documents in a storage locker in the basement of Mar-a-Lago. The feds have been aware of this for some time, but were hesitant to go after him aggressively for a lot of obvious reasons. They had been working with his attorneys for a while and did manage to get the really top secret shit back, but there were still things that Trump was keeping from them — apparently, he didn’t respond to a subpoena about it just a few months ago. So Garland just said fuck it and went after him because Trump is violating the law.