What does every investor know that every entrepreneur should know? It is the one thing that investors relentlessly hound entrepreneurs about, it is the simple question of "Who the fuck cares?"
If you can't answer that question as an entrepreneur, you have a lot of work to do. The reason that investors know this and many entrepreneurs don't is because investors see many startups and watch the 50-75% of their investments fail. Regardless of whether they take a methodical approach to building or evaluating the business if no one but the founders care the business is going to go nowhere.
If you're an entrepreneur...the more evidence you can bring about people caring (buying, using, and so on), the harder it will be for investors to gloss over the business.