1) the crossing or decussation is partial and does not involve all the fibers, 2) for the coordinated and smooth function of the whole brain, the various parts need input from the opposite side. this results in feedback and feedforward loops. if this "talking" between the two opposite parts is not present then the two sides of the body will function in a way that cancel each other's action. an example will be when walking both the legs will step forward. one easily see what the result of such an action will be. 3) this crossing of the fibers help in the recovery of a part that is injured/damaged. this has been seen in studies using fMRI in patients with strokes. if one side is paralyzed and the patient is asked to move the good side, then there is activity in the area that suffered the damage causing the paralysis. hence, by using the normal side, the brain can repair the damaged area. unfortunately, the repair is not always complete.

these are just a few reasons why the decussation is required. there are many other reasons which I will not go into as I think the above has given you an idea of the importance of the crossing the midline of the fibers in the CNS. thanks.

Thanks for your explanations. But as to number 2, do you know whether people with split brain (corpus callosum sectioned) have problems in performing coordianted and smooth functions?

See thread below:

https://www.researchgate.net/post/What_is_the_evolutionary_advantage_of_having_ones_brain_left_hemisphere_control_the_right_side_of_the_body_right_hemisph_control_the_left_side