It used to be that husbands always out-earned their wives. Many times, these husbands went to work while the wives stayed home, raised the children and ran the household. Due to this arrangement, when there was a divorce, it was the man who would be paying his ex-wife alimony. The idea was that this money was due to the fact that the woman had sacrificed her own earning potential to maximize that of her husband's.
However, looking at marriage and divorce today, much has changed with one of the largest differences being who is the breadwinner in the house. While it used to be the man, now there are plenty of family situations where the woman is out-earning her husband. In some cases, this means the wife might be the one working, while husband stays home with the children. In other cases, it is that the husband's earnings are supplemental to his wife's.