Iron is used by the organism to make hemoglobin, the molecule that transports oxygen in blood. Many recipients of blood transfusions need it because they lost

Iron is used by the organism to make hemoglobin, the molecule that transports oxygen in blood. Many recipients of blood transfusions need it because they lost a lot of blood: car accidents with open fractures, gunshots, etc. A single blood transfusion won't restore their blood levels to normal; it's just a temporary fix to keep them from dying. When they're "out of the woods", they restore the rest of their hemoglobin levels on their own, in time. It takes a lot of iron to do that, so the extra iron helps a lot. The problem with extra iron happens when the patient can't fabricate their own hemoglobin, and as a consequence consume almost no iron. Some of these disorders require repeated transfusions; the patient can't live without them: thalasemia (severe forms), porphyrias, refractory anemias (MDS), aplastic anemia. If the hemoglobin is low, the body's chemical mechanisms assume there's insufficient iron. The body then absorbs more iron from food and does it's best to store it and not lose it. In these cases, hemochromatosis appears even if blood transfusions they receive contain regular iron levels.