The always incisive and effervescent April Dunford has a great post at her blog Rocket Watcher about the reasons why your start-up needs a website, before you have a product to talk about. I couldn't agree more, for the following reasons:
Customers are buying your business model, not just your product. Your product is one source of value to potential customers, but not the only one. A good case in point: the many SaaS start-ups that attracted a lot of initial attention, then quickly lost their users to competitive products. If there's more to keeping your customers than just the product, you'd better start communicating those other sources of value.
You need the opportunity to test your marketing. You won't get the communication right the first time, so market testing before the moment when you release the product is critical. Who wants to slave over a great new invention, only to discover that your marketing sucks?
You need early product feedback. All new products are based on a set of assumptions, some of which will be wrong. You can use your website to understand your potential customers better through the website. If there's something wrong with your personas, use cases, or any other guiding principles of prioritization and design, identify the problems now, unless you really enjoy re-engineering.