I would guess that they want to simulate a percentage of the shock force of a near miss or hit from a (russian, chinese, other equivalent-tech) torpedo or naval mine without actually risking rupturing the hull. So they need a much greater weight of explosives positioned a much further distance away than if they were to actually fire a torpedo at the ship.

Or for general shake and vibration and shock force testing of the entire ship, simulating a combat environment. Unlike the shake/rattle/hydraulic ram rigs which are used to qualify a new airliner design on a structural test article, there's no other way than lots of explosives to shake/vibrate an entire Nimitz, Ford class size aircraft carrier.

Does it need to be the direct analog of any specific weapon, to be a useful test?

Almost every decom’d vessel that gets blown up for RIMPAC is turned into an artificial reef.