Prior to Islam, men used to marry as many women as they wished without any limits nor conditions. When Islam was revealed, it prescribed a limit to the number of women one may marry and also placed conditions for this to take place.
As for the limit, Islam prescribed that the maximum number of women a man can marry is four, as stated in the Quran: "Marry women of your choice, two or three or four" (4:3). When a man from the tribe of Thaqeef who was married to ten women, embraced Islam the Prophet Mohammed commanded him to choose four from amongst them and to divorce the rest.
As for the condition, it is the in the confidence of the man that he can actually be totally just and fair between his wives, otherwise he is not allowed to re-marry. In addition, the other conditions of any marriage must also be present, such as the ability to provide for the family and the ability to satisfy the sexual needs of the woman.
The reason for the allowance for a man to marry more than one woman is because Islam is a realistic religion and one which is not based upon idealistic notions which would cause real problems of everyday life without solution or treatment. It is very probable that a man marrying a second wife could be solving a problem, in that his first wife is incapable of bearing children or has extended menstruation cycles which result in his sexual needs being unsatisfied. The first wife could be ill and thus, instead of divorcing her and leaving her alone, could marry a second wife and remain next to his first wife, and so on. This allowance also solves the problem of a widow who needs a husband to care for her but does not wish for an unmarried young man, similar to a divorced woman with children. Indeed this allowance may solve a social problem which arises from the high proportion of good women who want to marry in comparison to able men. This is a common problem which increases particularly in the aftermath of wars and the such. The fact, in this case, is that the extra women do one of three following options:
1) That they remain unmarried for the rest of their lives, and are thus deprived from being a wife and a mother, which is a great injustice.
2) That they fulfill their sexual needs regardless of decrees of religion and acceptable behavior, which will result in a tragic loss in this life and the hereafter.
3) That they agree to marry an already married man who is capable of meeting their living and sexual needs and who is confident in his ability to deal fairly and justly between his wives.
As for those who say that this allowance is often abused by some men, it is an unfortunate fact that many rights are abused or are used in inappropriate manners. This does not mean that we must cancel these rights. Indeed, there are many men who abuse their first and only wives, so does this lead us to cancel marriage in its entirety? Freedoms are often abused. Should we cancel freedoms? We see that states and governments abuse elections; would it be right to cancel these processes? In fact we find that authority and government is frequently abused, so would it be acceptable to cancel authority and let society decline into a state of chaos? It would be better, instead of calling for the cancellation of these rights, to set up boundaries and regulations which would limit the possibility of such rights being abused.