Say the Japs had DEI & Phil, and its main force is at Carol Is. Ideally if you could get 5 allied subs in range you could spread them out in 5 different sz all at once costing Jap $ (8-10 ipc) and forcing them to buy more DD’s to go after you. At the very least it could be a stall tactic or weaken the Jap force. Jap would need to make a choice to attack your main force w/o DD’s (cheap causalities), or leave the convoy raids in place. If Jap goes after the subs, you may have created targets of opportunity to go after.