The whole idea of Creation is the struggle between good and evil, and to show the difference. If there was no darkness, we wouldn't appreciate the light. If we didn't have the Devil, we wouldn't appreciate God and Jesus! There has to be evil in order to understand good. There has to be the opposite side of the coin to show the difference.
The Lord had to let Adam and Eve fall in the Garden and let them eat of the Tree of the Knowledge of Good and Evil, so they would know the difference. They had all the good, but they may not have understood what good was, and how good it was, because they didn't have anything to compare it with, it was all good. They never really appreciated the Garden until they lost it.
There has to be a little evil and a little bad to make you appreciate the good things. If you didn't have a few troubles you wouldn't thank the Lord for all the good things! If you didn't have problems, you wouldn't appreciate the solutions. He allows the bad or the evil to show you that you never had it so good, and to help you be more thankful for when things are good, and to help you appreciate Himself--the Good of all goods!