Anchor Text and Links popularityAnchor text are those words within the clickable part of any link. In addition, the search engines link popularity algorithms also take into a count the age of the link. Meaning that older links will mostly have more power than the newer ones. Search engines also try to take into consideration that how relevant the content on the page that links to yours is, but this is difficult for them to do as its very subjective. Search engines put at least as much weight in what other sites say about your link popularity as they do in what you say about your own site through your content. The main reason they do this is, in very simple terms, because you can't be trusted.
Human interaction with Search EnginesIt's human nature to try to gain every possible advantage from the search engines and because of that some people try to trick them with the information they put on their pages. In other words, they spam the search engines.With link popularity as a major component and a search engines ranking formulas, it adds an additional layer of protection for them. Traditionally, when someone links to a website, they did it because they like the website and wanted to recommend it to their own site visitors. Therefore, the sites with the most links will likely be the the best sites. Links to search engines are considered a "vote". After all, when someone links to something that was "icky", the bottom line for search engines is that lots of links pointing to a site is a signal that the site is probably good.
Unfortunately, as link popularity became such a large factor in search engine algorithms, like On Page SEO, it too got gamed. This makes it even more difficult from many websites to get links because people are sometimes linking for the wrong reasons. That is, they're linking just because someone else will link back to them, rather than linking because they like the site.