he did it to teach Satan a lesson, and the angels that fell with him
that flawed, imperfect humans could learn to follow God through faith, instead of sight which Satan had
Satan and the angels saw, spoke with, and heard from God face to face. When they sinned, they did such a stupid thing, God taught them just how wrong they were by using imperfect created beings, humans, to show His love
Humans respond by faith. To show love to people who never even saw God, that proves God is love
Seems like mankind has paid a heavy price to make a point to Satan and his followers.
God created humankind because He was lonely. He created us in His image, to be "like" Him on some level in many ways. Angels and Satan are not created in His image; they cannot relate to God. Humans, by the very act of being made in God's image can relate to Him. Therefore, how could God not allow Adam and Eve the choice to sin? There is no true "relationship" when one party comes to the relationship by force or by lack of other options.