Without getting too deep into Christian Dogma, Catholicism maintains that God expressly gave free will to humans so man is left to freely choose the right path, and this in turn makes God's creation of humans valid, as opposed to a selfish act of vanity (Making creatures that have to and can only love and worship you).
Adam and Eve, however, is an allegory about the importance of obedience. The devil represents temptation, obviously, and the point of the story is really - "Listen to your parents (elders, superiors), or bad shit will happen." Parents, elders, superiors, and in this case, God, create guidelines so you don't accidentally kill yourself and learn to function well within the society you want to exist in. The difference between the Old Testament and the New Testament is what those guidelines are meant for.
In the Old Testament, it was quite literal, a set of rules that helped you not to die in the society and times it was written in. It is widely accepted among religious scholars that most of the Old Testament is not expressly the "word" of God, but rather the intentions of a loving, fatherly Creator working through the interpretations of the religious leaders of the time. I.E., God wants you to be happy and healthy, and I, as Abraham, have learned some things that will keep you this way - don't ingest certain types of food, don't kill other people, and don't lie with other men to keep our numbers up and people stop enslaving us.
In the New Testament, the guidelines are about emotional and social well-being. The allegories start to be about sharing, caring, and loving the rest of humanity as much as possible. It's more of a "one-size fits all" mentality meant to promote understanding, self-sacrifice, and acceptance of others.
Adam and Eve is the first allegory in the bible because it's the first thing we learn as humans - to trust and obey the judgment and wisdom of those responsible for our care.