For a long time, the link building process was placed on par with SEO in general. People would generate many links leading back to their site, hoping it will contribute to better results in Google. And it worked, at least for some time.
The year 2013 and the Penguin algorithm’s implementation brought a solid crush for those who invested in buying large amounts of links from random (and cheap) sources. Google algorithm started to– quite honestly– recognize the quality of these links as completely worthless. With that change, links’ quality became essential for creating a successful SEO strategy, and link building itself is now one of its pillars.