The Fall created a situation which formed a gulf between God and his people. With the advantage of having Nephi's writings on the small plates we can better understand the reason for this great gulf: "But behold, I say unto you, the kingdom of God is not filthy, and there cannot any unclean thing enter into the kingdom of God…" (1 Ne. 15:34)
The Fall of mankind sets in motion a situation which required a resolution. If something were not done, mankind would be irretrievably lost. Once they had sinned, they would be forever cut off from God. God would send his children to this earth, and they would unavoidably be lost and become subject to the devil.
Of course this is inconceivable, and there must have been a plan to redeem mankind from the beginning. Indeed, Christ as the lamb slain from the foundation of the world (Rev. 13:8) is the sacrifice who removes the sin from all the world and allows the reconciliation. It is in symbolic enacting of this event that the performances of the law of Moses become a type of this Christ to is to come, this atoning Messiah (see Abinadi's argument that the law is a type for Christ, Mosiah 13:10).