One of the most important aspects of operating a website is the ability to convert random traffic into customers or other registered users. It's infamously difficult to get people to take a minute or two to register on a new site. After all, most people are already members of several social networks, they probably have accounts at at least one or two of the major retail sites and no doubt a smattering of other registrations. It's hard to convince them to sign up to yet another site.