Where did we come from and what was the purpose of our creation. Are we truly the family of God or a random creation that came into existence at our birth created out of nothing in a process that only only started 6000 years ago? What is the worth of a human soul and what is this soul?
While it is true they don't understand it, is isn't true that it can't be understood. God can and does reveal his purposes. It also would make sense that if something can be created without prior existence as if summoned from nothing it can be uncreated by the same means.
Through this life's opportunity and the powers of a redeemer we can eventually gain perfection of the body, the spirit or both to the degree that we are willing to submit this eternal will that God has given moral agency to.
In this way we keep some autonomy and don't make choices to get a reward or to avoid punishment. Our judgement comes later after we have shown that independently we chose good or evil. In this way we are free and find out if we will submit our will to God or to some other power.
Doctrine and Covenants Section 88