Answer:
From a religious perspective, the advantages-as seen by those who believe it to be a commandment of God-are having a place in the Celestial Kingdom (i.e. the highest heaven).
If it's practiced for spiritual reasons-as opposed to lustful reasons-it also develops a spiritual and emotional closeness amongst family members that cannot be mirrored any other way. It also helps to develop within an individual the ability for the removal of negative emotions such as jealousy or selfishness.
It also creates a condition to be able to fulfill the commandment given to Adam to "multiply and replenish the earth." It also creates an environment where more than two spouses can work together in supporting a family, as well as having more family members to associate with and rely on.