A persistent SEO myth is that once you have optimized a site for SEO, the job is done. This was true before Google but everything changed whem Google introduced "Google Page Rank", its patented search algorythmn. This is not named for "ranking pages" but for its author, Google co-founder Larry Page. This algorythmn incorporated not just "on page factors" in determining search results but added weighting for how many inbound links pointed to a site and also measured how "important" the sites were that the links were from. From then on, SEO professionals have needed to pay just as much attention to recruiting inbound links (sometimes referred to as "link juice") as to optimizing site pages.