Affiliate marketing is the art of marketing products, services or information for others. It doesn't require that you ever house or warehouse a single thing. But it does require that you have an audience to market those things to online. Without that audience, whether it's through search engines like Google or social media channels like Facebook, you'll find a difficult time with affiliate marketing.
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.
Security. With all the malware and viruses online, Google has decided to include security as one of the factors in web site ranking. This covers two important issues: One is the SSL certificate mentioned earlier. Another is insuring your website and servers are up-to-date and free of infection. You'd be surprised at how many bots are poking at websites looking for vulnerabilities. Google will pull you out of search results if your website is compromised (although you can ask for re-ranking once you clean up your site).
Twitter allows companies to promote their products in short messages known as tweets limited to 140 characters which appear on followers' Home timelines.[33] Tweets can contain text, Hashtag, photo, video, Animated GIF, Emoji, or links to the product's website and other social media profiles, etc.[34] Twitter is also used by companies to provide customer service.[35] Some companies make support available 24/7 and answer promptly, thus improving brand loyalty and appreciation.

What's the authority of your website or webpage, or any other page on the internet for that matter where you're attempting to gain visibility? Authority is an important component of trust, and it relies heavily on quality links coming from websites that Google already trusts. Authority largely relates to the off-page optimization discipline of SEO that occurs away from the webpage as opposed to the on-page optimization that occurs directly on the webpage.
You can confer some of your site's reputation to another site when your site links to it. Sometimes users can take advantage of this by adding links to their own site in your comment sections or message boards. Or sometimes you might mention a site in a negative way and don't want to confer any of your reputation upon it. For example, imagine that you're writing a blog post on the topic of comment spamming and you want to call out a site that recently comment spammed your blog. You want to warn others of the site, so you include the link to it in your content; however, you certainly don't want to give the site some of your reputation from your link. This would be a good time to use nofollow.
The world is mobile today. Most people are searching on Google using a mobile device. The desktop version of a site might be difficult to view and use on a mobile device. As a result, having a mobile ready site is critical to your online presence. In fact, starting in late 2016, Google has begun experiments to primarily use the mobile version of a site's content42 for ranking, parsing structured data, and generating snippets.

Digital marketing is defined by the use of numerous digital tactics and channels to connect with customers where they spend much of their time: online. From the website itself to a business's online branding assets -- digital advertising, email marketing, online brochures, and beyond -- there's a spectrum of tactics that fall under the umbrella of "digital marketing."
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