Our Heavenly Father knew we could not progress beyond a certain point unless we left Him for a time. He wanted us to develop the godlike qualities that He has. To do this, we needed to leave our premortal home to be tested and to gain experience. Our spirits needed to be clothed with physical bodies. We would need to leave our physical bodies at death and reunite with them in the Resurrection. Then we would receive immortal bodies like that of our Heavenly Father. If we passed our tests, we would receive the fulness of joy that our Heavenly Father has received.
Our Heavenly Father called a Grand Council to present His plan for our progression. We learned that if we followed His plan, we would become like Him. We would be resurrected; we would have all power in heaven and on earth; we would become heavenly parents and have spirit children just as He does.
We learned that He would provide an earth for us where we would prove ourselves. A veil would cover our memories, and we would forget our heavenly home. This would be necessary so we could exercise our agency to choose good or evil without being influenced by the memory of living with our Heavenly Father. Thus we could obey Him because of our faith in Him, not because of our knowledge or memory of Him. He would help us recognize the truth when we heard it again on earth.
At the Grand Council we also learned the purpose for our progression: to have a fulness of joy. However, we also learned that some would be deceived, choose other paths, and lose their way. We learned that all of us would have trials in our lives: sickness, disappointment, pain, sorrow, and death. But we understood that these would be given to us for our experience and our good. If we allowed them to, these trials would purify us rather than defeat us. They would teach us to have endurance, patience, and charity.
At this council we also learned that because of our weakness, all of us except little children would sin. We learned that a Savior would be provided for us so we could overcome our sins and overcome death with resurrection. We learned that if we placed our faith in Him, obeying His word and following His example, we would be exalted and become like our Heavenly Father. We would receive a fulness of joy.