No-follow Links and SEO
The main reason for making a backlink no-follow is to avoid redirecting authority to another website. Because of this, the value of the page is not reduced when an outgoing link (read: backlink) is made no-follow. No-follow links are mainly used to refer to other websites. In addition, Google appreciates if you want to make a reference within your own site (read: internal link).
People also gather no-follow links to keep the total link profile in the proportion of do-follow links and no-follow links; this helps to maintain a natural ratio. A link profile wherein more than 95% of the total number of backlinks are do-follow indicates link buying. This is not appreciated by Google. As a website, you actually can get a penalty for this. Many natural link profiles do not have such a high percentage. So, one obtains no-follow backlinks for the quality of the link profile for deliberate reasons.