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.
Social sharing and comments: If you're on social media, you're probably familiar with sharing and commenting. Social shares and comments are good indicators of how relevant your content is with your target audience. If a viewer watches your video and takes the time to share it with their network, you probably created a great piece of content. Social shares are also important because the more times your video is shared, the more it'll be viewed. If your goal is to reach to reach a lot of people, social shares is good metric to track. https://buzzingofferdigitalmarketing.tumblr.com/