Website saturation and popularity, or how much presence a website has on search engines, can be analyzed through the number of pages of the site that are indexed by search engines (saturation) and how many backlinks the site has (popularity). It requires pages to contain keywords people are looking for and ensure that they rank high enough in search engine rankings. Most search engines include some form of link popularity in their ranking algorithms. The following are major tools measuring various aspects of saturation and link popularity: Link Popularity, Top 10 Google Analysis, and Marketleap's Link Popularity and Search Engine Saturation.
So if you think about it, SEO is really just a process of proving to search engines that you are the best site, the most authoritative, the most trusted, the most unique and interesting site that they can offer to their customer - the searcher. Get people to talk about you, produce good quality content, get people to link to you, and Google will be more confident that you are the best result that they can offer to their searchers, and that’s when you will start ranking on the first page of Google.
Additionally, there are many situations where PPC (a component of SEM) makes more sense than SEO. For example, if you are first launching a site and you want immediate visibility, it is a good idea to create a PPC campaign because it takes less time than SEO, but it would be unwise to strictly work with PPC and not even touch search engine optimization.