1098 Hours of Instruction

The state of Michigan requires that schools offer students 1098 hours of instruction each school year. These hours equate to around 180 days of school. I am sure most states have a mandated minimum amount of time for instruction. WHY? How are these hours calculated? Why are we concerned with how many hours are offered? NOT what was learned or how many hours the students was actually in class? Do all students learn at the same rate?