As already said, bringing home a healthy sub and crew is almost foremost; sinking tonnage is the priority, but getting your crew killed in the process does not further the war effort, unless it is a spectacular target. For example, Prien did not think he would escape from Scapa Flow, but BdU was willing to sacrifice one sub for such a coup. His escape was a bonus and he actually got to see his Knight's Cross rather than have his widow pick it up for him.
At a certain point in the war it was a death sentence to go to sea. The Allies had become that good. With a heavy heart, Doenitz ordered his boats to tackle the Channel during Overlord. He knew the chances were slim, but sinking one ship could prevent much materiel from getting ashore and facing the Wehrmacht.
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