President Joseph Fielding Smith showed how the book of Moses helps us understand why the Lord commanded Adam to not partake of the fruit:
“Just why the Lord would say to Adam that he forbade him to partake of the fruit of that tree is not made clear in the Bible account, but in the original as it comes to us in the Book of Moses it is made definitely clear. It is that the Lord said to Adam that if he wished to remain as he was in the garden, then he was not to eat the fruit, but if he desired to eat it and partake of death he was at liberty to do so” (Answers to Gospel Questions, comp. Joseph Fielding Smith Jr., 5 vols. [1957–66], 4:81).