Answer: Because if the egoistic evil inclination that is completely detached from the spiritual world didn’t exist, then also we and all of creation that is the opposite of the Creator would not exist.
If the creation will rise to the level of the Creator, then it will have some additional value. Its nature is bound to be the opposite of the higher nature, otherwise it would not be independent.
The descent of the worlds from above to below, the breaking of the soul, the manner of our existence in this world, and everything that we experience was done by the Creator so that we will feel at least a little of what the evil is that exists within us that the Creator created.
We can access this evil gradually, and through its transformation to good, we climb the 125 spiritual levels.
From the Kabbalah Lesson in Russian 2/21/16