Why does the Church of Jesus Christ of Latter-day Saints perform baptism by proxy for deceased people?

The Church of Jesus Christ of Latter-day Saints performs baptism by proxy for deceased individuals because the church views baptism as an essential requirement for salvation.1

This practice is specifically intended for those who died without having the opportunity to learn about or accept the faith during their lifetime. By performing these rites by proxy, members of the church seek to offer the deceased the same opportunity for salvation that is available to the living.1

Because of this theological commitment to saving the deceased population of the earth, the church places a significant emphasis on genealogy. Members compile meticulous records to identify ancestors and others who may be candidates for these ordinances, which are conducted exclusively within the church's temples.1