Columnist Jack Colwell (South Bend Tribune, Indiana) did write up a piece on possible reasons: Revenge. Blackmail. Profit.
I don't think any one of those is a stretch as motive. On the other hand the columnist didn't provide evidence.
A lot of people care about the issue, some because of what he took, and others because it was taken back.
www.southbendtribune.com
Honestly I don't think that anything is absolutely beyond the pale for Trump. When he wants something, and there are legal obstacles, he tries to have his way anyhow, or get someone else to work around the situation.
Revenge: his own sitting government ticked Trump off by essentially making him leave when his term in office was over. He wanted to stay, and couldn't make it happen. So maybe he took sensitive government documents to Mar a Lago in sheer rage and "because he could" -- figuring he'd decide what to do with them "to get even" later on. He always seeks revenge when he feels he has been crossed. Ask anyone who had worked for him, or with whom he has had some dust-up in Congress or out in the corporate world.
Blackmail? Trump is on record saying in a press conference once that he might "have something" on an unnamed US Senator, but would hang onto that in case he might later decide to write a memoir. And really who doubts that Trump "has something" on Lindsey Graham.
Profit? Some of the documents seized at Mar a Lago apparently had to do with intel on French president Macron. You decide if Trump would have been more likely to use that info to blackmail Macron for money, or just to try to get some real estate easement or something in a pricey part of that country. Why would he take that information with him if not meaning to use it somehow in future? Some of the other sensitive documents might conceivably be used to get money or favors from foreign governments. But If there's proof of that, it's not yet been confirmed to the public.