For a long time now search engines have been using links as votes, which
represent the web’s opinion on what pages users find relevant and useful
based on popularity. After years of programming development, the search
engines have mastered and refined the use of link data, where complex
algorithms are put in place to create nuance evaluation of websites based
on this information.
Links are not the entirety of SEO, but most search experts agree that huge
portions of search engine algorithms are designed around link-based
factors. By using links, the engines can determine a site’s popularity in a
specific niche based on the number of pages linked to it and when this
information is combined with other metrics like trust and authority, the
search engines assign a rank for the website.
Trustworthy sites usually link to other trusted sites while spammy sites get
the fewest links from trusted sites and links are a great way to establish the
validity of documents on any given subject.