Imagine a class which is split completey regarding previous knowledge which is in some way needed for the class.
How can you deal with such a class? How can you - without giving too much workload to those without previous knowledge - bring those students to the level which is needed in order to understand new things - without bothering those who know this already?
(Background: In my university, the future high school teachers don't have to take a class in real analysis and measure theory. But they have to take a class in probability theory together with the bachelor students who did take the real analysis class. Of course you don't need to know everything about measures for the probability theory class, but some background is more than helpful. How much content and methods of measure theory would you repeat?
Another example is a course about partial differential equations where some students might have attended classes in real analysis and functional analysis; and some students haven't attended either of them. Here, you have usally very motivated students who want to take the course but they were not informed in the past.)