My mom beat cancer but now needs a hyterectomy, why?

My mom had cancer a few years ago and had a masectomy of her left . she also had chemo and radiation and has no reoccurences. The doctor suggested she has a hysterectomy and salpingo oophorectomy (removal of ovaries). She does not have cancer of the ovaries or cervix and her doctor said it is a prophylactic surgery ( preventative). Why is this?