1. It is important that we DO believe in a pre -mortal existence where we lived in the presence of God because, unlike mainstream Christianity, we need to realize that just making it to heaven is not the ultimate goal. If we started out in heaven with God, from the start, then the design is to not just return but to return better than we left. The infinite reward makes risking hell worth it.
2. It is important that we DO NOT believe in multiple lives or reincarnation because we must understand that "the day of this life is the day for men to perform their labors". We can't procrastinate and get serious on the next "go'round."
32 For behold, this life is the time for men to prepare to meet God; yea, behold the day of this life is the day for men to perform their labors. (Book of Mormon, Alma, Alma 34)
3. While we DO believe this stage of mortality is our great and final exam, and there are NO second or third chances to "have-a-go" at mortality, how is eternity still described as "one eternal round?" What is it that "goes around"?
The thing that "goes around" in the universe is the new spirits that are prepared to come to an Earth and have a mortal/physical experience. During this experience, spirits must learn or are tested in 3 areas: 1. will they love and serve God over Satan. 2. will their spirit master and control their body or will they instead be controlled by their appetites and even genetics (natural man). 3. will they humble themselves and submit/consecrate to God's earthly priesthood authority. (this attitude = charity, opposite = pride)
The reason Why it is critical to learn these lessons NOW during this lifetime is because you cannot learn these lessons later. And if you have not or did not learn these 3 lessons in life, we cannot teach these lessons to future generations of spirits who will come to their Earth and have their "go'round". We won't be able to give future spirits their "first lessons". Those who never figure out mortality won't be enabled to teach "mortality prep".