When do you start promoting your products?

I'm creating a membership product for customers to learn electrical engineering to get a better job. Some of the people I have talked to asked why I would start promoting the program before it was even ready to sell. The biggest reason for me anyway is that if I waited until it was done, I would constantly find more stuff to keep adding or changing and would never finish. Instead, I am planning on a membership plan so that I will be able to work on the product while it is being actively used by customers. This should also lead to a better product through feedback.

But I know there are many people who insist on completing a business product before being comfortable even talking about it. Is this you? Why? What is your reasoning for this?