On top of what everyone else has listed ("You have to know the material well" etc) if you're writing material for the course, it's good practice for writing documentation, because documents - be they course notes or technical documents - are written to transfer knowledge, even in industry.

But when you're in industry, often the documentation is for someone you haven't met and won't meet, so it's easy to fall into the trap of documenting what you did and not what the stakeholder/reader needs to know.

When you're writing for students in your class it's easier to practice and develop that mindset of useful documenting, since you're in close proximity to your stakeholder/reader. Hell, if you're really lucky a student will put their hand in the air and tell you your notes suck, and then you've learned something you can improve on.