Google looks for authority when ranking web pages for a specific keyword. One of the ways that Google determines whether or not a site has authority is by evaluating backlinks.
If major websites like The New York Times, Huffington Post, and BuzzFeed link to a site as a source of info for a particular subject, Google will pass pagerank to the linked site. As a result, that website will more likely get a high rank for search terms related to its subject.
That’s why it’s important that you build a healthy backlink profile from credible sites with high domain authority.
Because SEM mainly focuses on the paid side of search marketing, backlinks don't carry the weight that they do with SEO.