Sperm washing is a requirement for IUI. It is not possible to use sperm in its "raw" form with IUI, because it contains prostaglandins chemicals that can cause uterine contractions. During intercourse, these prostaglandins are eliminated as sperm makes its way up from the vagina, through the cervix, and then into the uterus. With Intrauterine Insemination, these two "stops on the journey" are skipped, and that is why sperm washing is carried out. If sperm would be inserted into the uterus though IUI without first washing it, this could lead to strong cramps and contractions, and can even lead to uterine collapse. The sperm washing procedure also gets rid of dead and abnormal sperm. Once sperm has been washed, a woman's chances of getting pregnant increase significantly.
Sperm washing essentially removes everything that could interfere with fertility, and leaves only healthy and strong sperm. Who benefits from sperm washing? Everyone who will undergo IUI will also get sperm washing. But the procedure can also be beneficial in other situations. Couples who are trying to conceive while the male partner has morphology and motility problems can benefit, even without IUI. Those with antisperm antibodies may also benefit, and sperm washing has even been shown to reduce the chance a HIV positive man will transmit the virus to his partner, thus giving him the chance to have biological children without giving his partner and future child HIV. Women who have endometriosis can also increase their chances of getting pregnant after being inseminated with washed sperm.