Say there is School X and School Y.
School X marks very easily and getting 90%+ in assessments in really easy.
School Y marks very hard and getting 80%+ in assessments is an achievement.
The end product of the students in School X may be inferior to that of School Y as marking standards are often varied between schools. For that reason, it is difficult to take raw marks (what you get back after every assessment) and use that to calculate your final mark. The rank is used to weight your internal performance with your external performance thus getting a balanced result.