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.
Digital marketing is the marketing of products or services using digital technologies, mainly on the Internet, but also including mobile phones, display advertising, and any other digital medium.[1] Digital marketing channels are systems based on the internet that can create, accelerate, and transmit product value from producer to the terminal consumer by digital networks.[2][3]
Another challenge is the sheer scope and scale of digital marketing. There are so many great digital marketing techniques ranging from search, social and email marketing to improve the digital experience of your website. Our article, What is digital marketing? shows how by using our RACE planning framework you can define a more manageable number of digital marketing activities which cover the full customer journey. Within each digital marketing technique, there are lots of detailed tactics that are important to success, so they need to be evaluated and prioritized, for example from dynamic content for email automation, website personalization to programmatic, retargeting and skyscraper content for organic search.
Inbound marketing refers to a marketing methodology wherein you attract, engage, and delight customers at every stage of the buyer's journey. You can use every digital marketing tactic listed above, throughout an inbound marketing strategy, to create a customer experience that works with the customer, not against them. Here are some classic examples of inbound marketing versus traditional marketing: 
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