Why did God need to plant the tree of knowledge of good and evil in the garden of Eden in the first place?

Wood did not have any mystical powers, It was a symbol of God's sovereignty. Something similar as is flag of one country at the embassy or government building.

With that tree, God told the people: "Earth and everything else that I have provided you is mine and only if you obey me you will live forever. Otherwise, in the day you refuse to show obedience, you will start to die." (read Genesis chapter 2, paragraphs 15-17) It was a reasonable request - Adam & Eve could eat fruits from any other tree, eat to satisfaction.

God did not want them to take the fruit from the tree, he is not contradictory to himself. He wanted the people and their children see the tree every day and to remember that they must be obedient to God because he knows what is best for our own good.

The Bible is simple and not complicated. ;-)

You ought to understand that there were so many good trees and plants in the garden of Eden, moreover, there was the tree of Life. God never commanded Adam and Eve not to eat of the tree of life yet He warned them not to eat of the tree of knowledge of good and evil. Man was created as a free moral agent… their destiny was in their hands and unfortunately they CHOSE to disobey even in the face of a strict warning. Gen. 2:16 And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat: 17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die. That God intended for man to fall is tantamount to blasphemy. Our God is a good and loving God He will never lure man whom he has created in His image and after His likeness to destruction. It is a matter of CHOICE. God gave a similar commandment to the children of Israel in the wilderness. Deut 30:19-20.

If we study the book of Romans where it teaches that sin is the outcome of law, it meant that eating that particular fruit of the knowledge of good and evil was not a sin until God made it a law not to eat it. It also teaches that where there is sin, grace is much more abound.

As a Christian we all know that God is too wise to make a mistake and too loving to be unkind. There are no hidden agendas as to why god planted this tree in the garden. The answer is simple, God is loving but also Justice and how just can he be if he created mankind with free will and not given him the choice to choose between obeying or disobeying him? Law and Grace are two different covenants. The Law covenant that God made with Abraham requires grace whereas the Grace covenant that Jesus Christ made with the whole world does not require the law at all.

Your question really about the tree (to which I think screams being a metaphor) or is it about the nature of GOD and what his motives were/are?

A very basic tenet of Christian theology and of what we know of GOD through the self revelation in Jesus is that GOD is love and knows how to love without showing favoritism or with any dubious motive.

Would we who are evil set up our own children for such a fall and thus kick them out? No, of course not. Therefore, if we who are evil know how to be good to our children, then how much more so will GOD who is perfectly good and perfectly loving. Let's not bring GOD down to our level, or worse, make Him out to be more of a monster than we are. I am certain that the father in the parable of the prodigal son did not set his child up to be rebellious, and so I dont think Christian theology would say that GOD did the same to Adam and eve in the garden.

God planted the tree for the same reason he has done everything else on this planet.

So that he might be glorified all the more.

God is an intensely selfish God. He demands worship and honor and glory. He has created this world as a vessel so that he might be worshiped. He ordained the rebellion of Satan so that he might have an enemy and show his power in victory over it. He planted the tree in the garden so that he might have an imperfect people that he could redeem and show his power all the more.

God does not make mistakes, his creation was perfect by design, he allowed it to be corrupted by design, he has redeemed his people by design, and he will glorify his chosen people by design.

The tree existed to kick off the events so that God could show how much he loved his people and in that be glorified all the more. It allowed him to show his mercy and love to a fallen creation. He was gracious to them by not destroying them, and poured out his love by sacrificing his son. And in the future so that his son may return triumphant and bind Satan once and for all. The tree existed so that God would be glorified.