Long before the appearance of the Prophet of Islam(sa), polygamy was being practiced by the Biblical prophets. Abraham had three wives, David one hundred, and Solomon over seven hundred. It was a very common practice of the society to marry more than one wife. Polygamy was not only in Arabia rather it was being practiced in the entire world.
The Holy Prophet(sa) eliminated racial, ethnic, cultural, tribal and national distinctions and prejudices through his marriages. Some of the marriages of the Holy Prophet(sa) were to unite his followers with different clans and tribes in mutual love and affection.
At this time, divorced women were looked down upon as inferior in status and were usually not allowed to remarry. By marrying a divorced lady, the Holy Prophet(sa) taught that such a marriage is not only lawful but also desirable to grant a divorced woman an honorable status in the society.
Some of the Holy Prophet’s marriages were in order to wipe out certain prevailing evil customs and practices. For example, there was a common practice that an adopted son was considered equal to a biological son, and as such, was entitled to equal civil liberties, while the other close relatives were ignored and their rights were neglected. So Islam drew the line between blood relations and adopted children, and gave them proper rights as they deserved. In order to make the difference between the real children and the adopted ones clear, the Holy Prophet(sa) married Hazrat Zainab, the divorced wife of Hazrat Zaid, a freed slave and the adopted son of the Holy Prophet(sa). This way he established the correct status of an adopted child. Previously, it was not allowed to marry the widow or the divorced wife of the adopted son.
In the time of the Prophet of Islam, the prisoners of war were taken as slaves. The Holy Prophet(sa) set up an example by freeing them on ransom and on several occasions without charging anything. He also married two of the captive women, to set up a personal example for the believers.
Islam spread rapidly as compared to any other religion, and because a number of commandments and regulations concerning women could be propagated only through wives, so the wives of the Holy Prophet(sa) from different places and clans having diverse tastes, habits and temperaments, became helpful to educate the women folks properly.
Source: The Holy Prophet Muhammad (peace by upon him) by Maulana M.A. Cheema, Chapter: “Polygamy and the Prophet of Islam”, pages 13-15.