Why do x rays technicians stand behind a shield when giving xrays to patients?

to stop harmful radiation from damaging/mutating reproductive cells

to reduce (not eliminate) stray x-ray dose to parts of the body where such dose is undesired and not needed by required test. Note: significant damage to reproductive cells in a whole body dose would almost certainly be a lethal dose, you would not need to worry about mutations. But x-rays are almost never given whole body anymore, sources are collimated and aimed just where needed. Its the limited stray radiation that they try to shield against.