Briefly, We live our lives, and the kind of person we are remains with us. When we die, we don't hop straight to heaven, but instead await judgment. During this time, those who did not accept Christ in life (through ignorance or rejection) have the opportunity to accept him there. If they accept him, great! But they may have missed out on the ordinances that he commanded be performed for them, i.e. baptism, while on earth. Thus, in what I believe is not simply a workaround, but rather an opportunity for bonding and sacrifice on behalf of those who have gone before, those who still live and have a body can perform the ordinance for the dead. Now the dead accepts Christ, and with the proxy ordinance in place, is now read to face Christ for judgment. And the living proxy has, in his or her very small way, emulated the Savior by performing a saving act that someone else could not perform for him/herself. Hope that helps.