When a site is developed with a clear purpose in mind from the start, the everything about the site is designed to meet that goal. When the purpose is changed or non-apparent from the start, there will be problems down the road. Take the time to get the purpose right at the beginning of a project.
The purpose of the site or app shouldn't be vague. There should be a clear reason that it exists and there should be a goal in mind for the visitors to the site. Generally speaking, sites are developed to inform, educate, entertain, or conduct commerce.
Within those broad categories, sites and apps must identify the more specific reason for existing. There are plenty of existing options for information, education, entertainment, and commerce. The digital economy doesn't need a 10,000 Startbuck's anyway. Identify how a new site or app will add something new or do something better.
The purpose should be big enough that it would be impossible to know the outcome in a few weeks or even months. Small roadblocks should only stop small projects. Keeping the purpose big allows the site to develop and grow over time, with the same broader purpose in mind.