A major Search Engine Optimization company's newsletter I received today had this subject line: "When did you find out it was all about traffic?"
It didn't take me long to realize I had that figured out a while back. Generating traffic to your site is what increases sales. And sales are why you are in business, unless of course you are philanthropic.
Traffic also has a side effect. It raises page ranking because it proves you have good content, or what people are looking for. Good content plus high traffic (or backlinks) get you good grades in Google.