2 Nephi 2:25

"Adam fell that men might be; and men are, that they might have joy"

Adam and Eve were commanded not to partake of the fruit of the tree of knowledge, but they did. They partook so that they could know good from evil. They realized the importance of this knowledge and decided it was necessary to their mortal existence. Because they did, this they were able to have children and live a life where they could experience true joy. Living as a family is the best joy! Also, they were able to know evil, and knowing what is bad makes the good so much sweeter.