The Tree of Knowledge of Good and Evil is a rather mysterious tree. Aside from the age old question "what sort of fruit was it?" (contrary to popular cliché, scripture does not say it was an apple), many people ask why God would create such a tree in the first place. God not only knew it could plunge the world into bondage to sin, but that it would, so what gives?
As Christians, there are a few moral considerations we should make when asking such a question. It can get into the territory of blasphemy, as we might come to believe we know better than God, or even seek to blame Him for the fall. This is certainly the route taken by many atheists. Ultimately, God's thoughts are higher than our thoughts, and His ways higher than our ways. Thus, we have no right to think He should have done anything different, and no guarantee we will come to the right answer. However, this particular question may help us gain a deeper understanding of God, and so when asked for innocent reasons, I do not believe there is any sin in it.
The first thing we have to consider is what, exactly, evil is. I like the analogy that evil is like a donut hole. The hole cannot exist without the donut, but the donut can exist without the hole. In the same way, good can exist without evil, but evil cannot exist without good. Rather, evil is a corruption of good. It is entirely possible that the tree served some purpose in the pre-fall world. It may have simply been an aesthetic choice, but there may be other things God intended it to do. Perhaps animals could live in it, or maybe they were uniquely permitted to eat from it. Maybe it held something else in the garden in place, or kept something else at bay. Maybe it was particularly good for shade. It may have produced a special resource of some kind. For all we know, it may have even produced music when the wind blew a certain way. There is so much the tree could have done, even the very fruit, other than providing food for Adam and Eve.
But aside from that, I believe we put too much emphasis on the tree. I may be wrong, but I contend that the tree itself was actually inconsequential. Ultimately, God chose a tree, but He could have replaced it with anything He wanted. You are permitted to drink from any well, except this one. You may bathe in any river, except this one. You may sit on any rock, except this one. You may play with any animal, except this one. The point here isn't the tree, but obedience to God. The "thou shalt not" is the point! The tree merely provided opportunity for Adam and Eve to disobey.
To me, this makes perfect sense. In Romans 5:13, we read "For until the law sin was in the world, but sin is not imputed when there is no law." The tree of knowledge is described as being quite pleasing to the eye, and good for food. Had God never said a word, I believe Adam and Eve could have walked right up to it and stripped it bare, eating every fruit it ever produced, and nothing would have happened. But because God said "but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die", it immediately became evil for Adam and Eve to eat.
So we see that the tree is effectively inconsequential, because whether concerning a tree or not, God would have given Adam and Eve commands, as He has the right to do, and IS right to do. Therefore, regardless of the tree, we would still be in this same situation. Adam and Eve would have been in a position to disobey God, and they and their offspring would face the same curse we already do.
With all of this considered, we see that, first of all, God's plan wasn't so unusual after all. Things worked out exactly as He had planned, which is great for us, because ultimately, it means we get to experience His grace, and one day spend an eternity in His Kingdom. The question of why God created the tree in the first place is an interesting one, but it also shows us the extent of God's love for us, knowing that we would sin, yet setting in motion the perfect plan to redeem us. Knowledge of good and evil allows us to know evil, but it also allows us to know Him. Thank God for His glory.