EPO was very big in endurance athletics. More blood cells= more O2=improved endurance. Like blood doping. Testing makes this difficult to get away with in Olympic type situations, but I'm sure it's still done. I can refer you to someone who really knows his shit on this if needed.
My experience is more in the bodybuilding aspect, where the most obvious benefit is to give fullness to the appearance of the muscle. Pre-contest dieting and fluid manipulation can cause you to have a "flat" look.