Every visitor costs money. Whether or not these costs are immediately visible to you or are in the form of sweat equity and some low priced service providers, you are spending every minute your site is online. Every e-mail that flows into your inbox uninvited slows down the global network and costs time, money and resources. Multiply that by billions of unwanted e-mail messages and you've got a serious problem. The same goes for every individual web site.
Every visitor has the potential become a customer, or contribute to income generation.
Whether your site sells advertising space or a specific product it has the potential to make an income. Even sites not making the effort to generate an income still have the potential to make money if they have traffic. It's just a matter of choice. We therefore must conclude that every visitor has a value. And unless you are one of the pioneers of the Internet., or have the most original targeted site topic in the universe you will only be able to generate so much traffic without spending marketing dollars.