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.
I definitely learned tons of new things from your post. This post is old, but I didn’t get the chance to read all of it earlier. I’m totally amazed that these things actually exist in the SEO field. What I liked most is Dead Links scenario on wikipedia, Flippa thing, Reddit keyword research, and at last, the facebook ad keyword research. Its like facebook is actually being trolled for providing us keywords thinking they are promoting ads.
A breadcrumb is a row of internal links at the top or bottom of the page that allows visitors to quickly navigate back to a previous section or the root page. Many breadcrumbs have the most general page (usually the root page) as the first, leftmost link and list the more specific sections out to the right. We recommend using breadcrumb structured data markup28 when showing breadcrumbs.