M 3.9 Experimental Explosion – 147 Km ENE of Ponce Inlet, Florida

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.

The structure is engineered to survive a multitude of conventional threats intact. It is testing properties of the design rather than specific weapons per se. Also, these tests are intended to be non-destructive which impacts their design.

Exercises where the US military uses decommissioned aircraft carriers and other large ships as targets are illustrative. They are basically unsinkable. You can hit them with torpedoes, bombs, missiles, etc all day. At the end of the exercise they usually have to send over a specialized demolition crew to actually scuttle the ship. Astonishingly damage resistant.

A nuke would of course do the trick but now you are playing a different game.

People chronically underestimate how difficult it is to get enough conventional explosive on target to sink a major naval vessel, even ignoring the extensive active defenses.

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.