Polygamy is allowed not only in Islam but in other religions including Christianity and you can find it in Old Testament. Jews and Christians claim following Ebrahim (Abraham) and every one knows that Ebrahim had more that one wife and his sons Ismail and Isaac are from different mothers.
Polygamy is required by human societies in many situations especially when the number of marriageable females is more than the number of marriageable males due to many reasons. This situation will make many marriageable females in need for a man to marry them even if he is already married with another female. With out legitimate polygamy, thousands of unmarried females will suffer a lot.
Polygamy is a blessing from Allah (SWT) on the human beings especially on the unmarried , divorced or widowed females.
Islam does not grant open permission for men to have more than one wife but puts a condition of practical justice on the man who wants to have more than one wife. If the man is unable to maintain the practical justice between his wives, then he is not allowed to marry more than one wife (If you fear that you shall not practice justice, then just one wife) (Sura 4, Verse 3).