Possession of war are meant to increase the wealth of the master.
others can purchase the freedom of the slaves,
they can be sold to other master,
they can also be bartered with other slaves,
the slaves can purchase their own freedom
the master also can set free the captive of war if he will with no profit
Depending on how wealthy of a master they get, some of the possession of war do not even to be released from the custody of their rich master as they get taken care pretty nicely