The documents Trump had probably incriminate high ranking Dems in corruption or sex crimes...
Probably records of Clinton or Biden dirty business. Maybe Epstein-related. I can't think of anything else he would want to keep.
He had them for "insurance purposes". He wasn't going to release them because it would have likely resulted in widespread violence and chaos when the Left rioted in reaction. Trump probably didn't anticipate the raid.
They won't pursue anything against him because then they would have to reveal the contents of the documents. The purpose of the raid was just to get the damaging evidence out of Trump's hands and paint him in a negative light.