When would this be useful? If your site has a blog with public commenting turned on, links within those comments could pass your reputation to pages that you may not be comfortable vouching for. Blog comment areas on pages are highly susceptible to comment spam. Nofollowing these user-added links ensures that you're not giving your page's hard-earned reputation to a spammy site.

Disney/Pixar's Monsters University: Created a Tumblr account, MUGrumblr, saying that the account is maintained by a 'Monstropolis transplant' and 'self-diagnosed coffee addict' who is currently a sophomore at Monsters University.[73] A "student" from Monsters University uploaded memes, animated GIFs, and Instagram-like photos that are related to the sequel movie.
Optimizing a website may involve editing its content, adding content, modifying HTML, and associated coding to both increase its relevance to specific keywords and to remove barriers to the indexing activities of search engines.[citation needed] Promoting a site to increase the number of backlinks, or inbound links, is another SEO tactic. By May 2015, mobile search had surpassed desktop search.[3]
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