Actually, there isn't just one answer to this question, as breastfeeding is shown to have more than one advantage for the mother - and both a reduced rate of breast cancer and diabetes are observed, as compared to the mothers who did not breastfeed. It is possible that there are also other advantages - we don't know everything yet and research is still ongoing.
Some mothers can get very sick if they are not allowed to breastfeed, and it is speculated that Austrian Empress Elisabeth was sick for that reason.
