A submarine hunting merchant traffic has to find a moving convoy and close far enough to attack it. Escorts occupy the space between that convoy and the approaching threat. A submarine patrol that returns without engaging has not disrupted that convoy. Even a boat that remains undetected until it fires then gives the escorts a reason to intensify their listening and depth-charge response. That reaction is probable rather than a guarantee of detecting or destroying the submarine. The tactical advantage comes from making the attacker approach the ships already protecting its target.
Notable Point · 56:21-57:29
A submarine must find and approach the moving convoy
Convoy movement puts the burden of finding and closing on the submarine, while an attack can prompt nearby escorts to intensify their search.