SEM search placements include an “Ad” designation. SEO does not. Search results that appear as a result of SEM or SEO look different on SERPs. Paid ads that receive placement through SEM tactics are often identified as an ad (e.g., by an icon appearing next to the placement), whereas the search results that appear as a result of organic SEO are not marked in such manner.
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 engines are expanding - When someone mentions search engines, do you automatically assume they’re talking about Google? The tech giant has such a big share of the market that 'Googling' has become a verb. However, a significant portion of searches take place on alternative sites, such as Microsoft’s Bing. Make a point to search for your site on Google alternatives to see where you rank. Just improving social media engagement and adding meta tags might be all it takes to boost you a couple ranks on Bing.