How such a system came into existence is something which need research. perhaps different systems (matriarchal/patriarchal) prevailed in the societies might have resulted in this.
Another possible reason could be shortage of male members in the society or in a particular community. May be due to wars, which was very popular in ancient days, the death of men caused a drastic reduction in the number of male members. So parents of the girls were prepared to pay them for marrying their daughters !
There was a concrete example for such a situation. In olden days among the Kerala Brahmins (Nambudiris)only the eldest son was allowed to marry from the same cast. Others were forced to marry from other castes. So many Nambudiri girls could not get married. Hence the parents were prepared to give her married to old people (eldest son of a family who was already married) with some dowry. The eldest son was allowed to marry more than one girl in those days. thus by showing money such persons were attracted to marry second time or third time !
However such systems have completely gone from the society.
Gold Member, SPK