They're useful if you want to avoid escorts as much as possible. Launch a salvo of them ahead of a convoy, so that their ladder pattern runs perpendicular to the convoy's heading (so if the ships were heading east, the torpedo would make runs going north/south for example). The aim is to have the ships sail into an area covered by torpedoes running their search patterns. It's not 100% effective, but can produce good results and you don't need to target individual ships; meantime you can get some depth, reload in safety and come to periscope depth once the convoy's passed to pick off any damaged stragglers.
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