Every aspect of the plan of salvation revolves around agency. Agency has always existed; we had it long before our testing on earth. We know this from the war in heaven. When Heavenly Father presented His plan, Satan used his agency and presented another one, in which there was no agency, and Jesus used His agency to volunteer to be the Savior and glorify the Father. We then exercised our agency to choose between following Heavenly Father's plan or following Satan's plan.
Agency cannot exist, however, unless there are opposing things for us to choose from (2 Nephi 2:11-12). In the pre-mortal life, Heavenly Father's plan was opposed by Satan's plan. Those who chose to follow Satan were cast out and became the devil and his angels living in hell, the opposition to Heavenly Father, His angels, and heaven.
If Heavenly Father had sent Adam and Eve straight to a telestial world, it would not have been as a result of their agency, as every other part of the plan of salvation is: choosing the Father's plan to come to earth, choosing the right while on earth, and therefore choosing our eternal destinies. Although Adam and Eve had chosen to come to earth, they also had to choose the Fall.
Therefore, He created and sent them to the garden of Eden. Since there had to be opposition (v.16), there was the tree of life and the tree of knowledge (v. 15), the commandment to multiply and replenish the earth and the commandment not to eat the fruit of the tree of knowledge.
Now there was opportunity for Adam and Eve to use their agency. Satan tempted Eve and she chose to eat the fruit. Once Adam found out, he chose to do the same. Their use of agency resulted in the Fall: being physically and spiritually cast out from God's presence, which can only result from sin (or in this case, transgression), another reason why the Fall had to be chosen as opposed to being thrust upon them. It had to be the consequence of an action.
That consequence allowed for the need of the Atonement, the most important part of the plan. We now have the opportunity, through our agency, to use the Atonement and be brought back into Heavenly Father's presence.
Agency is truly the core of all gospel principles. Without it, everything loses purpose (vv. 11-12).