There are a lot of courses about almost every topic of importance. Have you ever looked at the number of books that teach something as simple as mx+b? The number of courses that teach about the different planets in the solar system? The number of courses that teach about Shakespeare?

Lots of people learn a lot of different ways. The more courses available for a single topic increases the likelihood that a learner will find a course that matches their learning style.

Writing courses is also a great way for someone to internally reinforce what they learned, and expose what they need to know more about.