Why is Hans' job in the military so undesirable?

Hans is sent to Essen, where he is given an undesirable job with the Air Raid Special Unit: his unit must stay above ground during air raids to put out fires, prop up buildings, and save people. Everyone in the unit had done something impolitic to get this assignment. Hans tells them about his having given bread to a Jew; the Sergeant laughs and tells him he is lucky to be alive.