The reason is that you usually start talking with the potential customers about the idea before you have a product ready. You bounce your idea from people on ba

The reason is that you usually start talking with the potential customers about the idea before you have a product ready. You bounce your idea from people on barcamps, you discuss the idea on forums, etc. Now - these people quite often want to know when you launch, and the best way is to give them the website address, so they can leave their e-mail address on it. So: promoting a landing page? Makes no sense. But talking about the idea before you have a prototype - definitely!