tracts of spine

please clear my doubts.
theres a reason for everything in the body.so why do tracts of spine cross over to contralateral side? there must be some advantage to the body by their cross over. so what is that advantage? what purpose does their cross over fulfill?

hi mahu how r u?ur from which place?how is life going on?u r question is good one, now i dont know answer i wil try to fond and msg u ok