Politics forum?
I grew up LDS.
As the scripture states, "Man must be born of water, and of the spirit" I believe it goes. Mormons take a literal meaning and think everyone must be baptized to live with God again (but it's not necessary to avoid hell).
So since there are people that never get the chance to accept/reject Jesus Christ, Mormons believe a just God will both satisfy the law, and give opportunity to those who were not afforded the chance. Proxy baptisms satisfy the law, and Mormons believe the person can choose to accept the baptism on their behalf.
It was practiced in Bible times, there is even a scripture referencing it. Interpretations vary. Corinthians 15:29