Why would athletes remove red blood cells only to return them to their bodies later? What problems could be created

Recently, athletes have begun to take advantage of the benefits of extra red blood cells, in a process known as “ blood doping“ (You can do your own research on it). Two weeks prior to a competition, a blood sample is taken and centrifuged, and the red blood cell component is stored. A few days before the event, the red blood cells are injected into the athlete. Why would athletes remove red blood cells only to return them to their bodies later? What problems could be created