What's testosterone used for in women?

Same as in men. Low testosterone levels in men and women can be returned to normal by testosterone supplementation in order to improve mood, thinking/memory, bone strength, sex drive plus erectile function in men. Testosterone supplementation can help increase muscle & decrease fat in those who also exercise & eat properly. Testosterone has similar risks in both men & women including heart attack & stroke, etc.
Off vs On Label Rx. On label use = not used off label use = to correct symptomatically low levels of testosterone. Testosterone supplementation and/or replacement therapy is usually a compounded rx that is often accompanied by a prescription of estrogen and/or progesterone.
Rarely. Tesosterone is very difficult to use in women because the required doses are so small and difficult to regulate. Testosterone in women will increase sex drive, but can have many very harmful consequences. Best to avoid.