TPM Reader TS has a different emphasis but a not dissimilar take to DP’s. Holding the documents is power, whether Trump actually uses them in some practical way in the future or not.
Look at it from his perspective: he’s still the rightful president and a bunch of wimpy gnats of bureaucrats are trying to take away presidential stuff he put away and brought to Mar a Lago. Why? Just because he might enjoy it or find it useful — almost certainly to defend himself against imagined wrongs or to get back at or hold leverage over his enemies at home and abroad (so many of them)!