Sounds like a nice essay question for an exam.
The short answer is that it contributes to uniform application of legal rules. Having a higher court with the ability to review a lower court does not mean that all of the decisions of the lower court will be correct, but it does create a means to correct those decisions caused by a lower court's misapplication of the rules or erroneous guess as to how the higher court would apply the rules when the rules are not crystal clear.
