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.
Search engine optimization is a key part of online marketing because search is one of the primary ways that users navigate the web. In 2014, over 2.5 trillion searches were conducted worldwide across search engines such as Google, Bing, Yahoo, Baidu, and Yandex. For most websites, traffic that comes from search engines (known as "natural" or "organic" traffic) accounts for a large portion of their total traffic.

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