> For example, if a course already exists on OCW, the value of having another course from another university seems very low. Why would someone pay for another archive of a course at another university?

This is where you are sadly mistaken. There is no canonical course out there! Every course is different; every teacher is different. Just because a course on, say, "Data Structures" exists, does not mean that every person who's interested in Data Structures can just watch that and find it equally informative. Many people may not understand the teachers accent; many may not like their teaching style (speed, delivery, stress, etc.). There are so many variables!