Lehi reemphasizes the most important facet of his discourse, that man is free to act. This action is critical, as the simple existence of opposition does nothing, but rather our actions in relation to it. Therefore, Adam and Eve were unable to exercise their agency, their ability to act, unless there were opposites available.
In the story of the Garden there is only one opposite. All things are permitted them. All things they are free to do. Therefore, with the exception of the Tree of Knowledge of Good and Evil, they have no "opposition" in Lehi's definition. Without the Tree of Knowledge of Good and Evil, according to Lehi, they would therefore be in a state where everything was a compound in one, where the really was no righteousness, no happiness, because there was no choice among the opposites. Thus the placement of the Tree of Knowledge of Good and Evil was critical to the plan of God. Without that tree and the opposition it symbolizes, there was no purpose in creation, using Lehi's logic. Even the name by which we know the tree indicates that without it Eden would have been Lehi's "compound in oNephi" The Knowledge of Good and Evil is an absolute precursor to the ability to make a choice between Good and Evil.