When evaluating a page to assign rank, search engines look at a host of signals of quality that might provide clues to what a page is about and the value of its content.
One such signal of quality that we can safely identify is the number of inbound links to a page from reputable sources. If a page has a number of good inbound links to it, then it is likely that it has high quality content and therefore should be ranked high when displaying relevant search returns.
This particular signal of quality is what allowed Google to rise to the top in the business of search engines while other search algorithms were looking at less reliable signals of quality. Obviously, looking at the right signals of quality is of utmost importance in the multi-billion-dollar search industry. [My comment: Wow! It makes sense now that I think about it, but until I read this I never really thought about “searching” as being such a valuable thing: “multi-billion-dollar search industry.“]
— Building Findable Websites by Aaron Walter