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.

“Syndicate carefully: If you syndicate your content on other sites, Google will always show the version we think is most appropriate for users in each given search, which may or may not be the version you’d prefer. However, it is helpful to ensure that each site on which your content is syndicated includes a link back to your original article. You can also ask those who use your syndicated material to use the noindex meta tag to prevent search engines from indexing their version of the content.”
Website owners recognized the value of a high ranking and visibility in search engine results,[6] creating an opportunity for both white hat and black hat SEO practitioners. According to industry analyst Danny Sullivan, the phrase "search engine optimization" probably came into use in 1997. Sullivan credits Bruce Clay as one of the first people to popularize the term.[7] On May 2, 2007,[8] Jason Gambert attempted to trademark the term SEO by convincing the Trademark Office in Arizona[9] that SEO is a "process" involving manipulation of keywords and not a "marketing service."
×