One teaches a class. One writes a sermon. One writes a blog. One writes an article. There is a difference between teaching and writing. When one is teaching, one can get feedback, one can get a response and respond to that, clarify, clear things up. You don't get that when you write a sermon, you don't get that when you write an article.
Here is the thought that I have. You can't be as advanced when "writing" as you can when "teaching" - because when you teach you can more easily make sure that things are clear. With teaching you can pull the students up through information - when writing the hearer/reader is left more on their own to sort and sift through the info.
When planning a class, you can plan at a different level of complexity than you can with a sermon. If you are writing an article for high schoolers to read it needs to be simpler than a class you would teach them face to face. That's just an issue of communication.
I'm not sure what the point is - but this is something that we should probably remember when we plan and write.