QUOTE: “Supplementary Content contributes to a good user experience on the page, but does not directly help the page achieve its purpose. SC is created by Webmasters and is an important part of the user experience. One common type of SC is navigation links which allow users to visit other parts of the website. Note that in some cases, content behind tabs may be considered part of the SC of the page.” Google Search Quality Evaluator Guidelines 2017

Linking to a page with actual key-phrases in the link help a great deal in all search engines when you want to feature for specific key terms. For example; “SEO Scotland” as opposed to https://www.hobo-web.co.uk or “click here“. Saying that – in 2019, Google is punishing manipulative anchor text very aggressively, so be sensible – and stick to brand mentions and plain URL links that build authority with less risk. I rarely ever optimise for grammatically incorrect terms these days (especially with links).
All of this plays into a new way businesses and SEO professionals need to think when approaching what keywords to target and what SERP positions to chase. The enterprise SEO platforms are beginning to do this, but the next step in SEO is full-blown content recommendation engines and predictive analytics. By using all of the data you pull from your various SEO tools, Google Search Console, and keyword and trend data from social listening platforms, you can optimize for a given keyword or query before Google does it first. If your keyword research uncovers a high-value keyword or SERP for which Google has not yet monetized the page with a Quick Answer or a Featured Snippet, then pounce on that opportunity.
Another reason is that if you're using an image as a link, the alt text for that image will be treated similarly to the anchor text of a text link. However, we don't recommend using too many images for links in your site's navigation when text links could serve the same purpose. Lastly, optimizing your image filenames and alt text makes it easier for image search projects like Google Image Search to better understand your images.
Many search engine marketers think who you link out to (and who links to you) helps determine a topical community of sites in any field or a hub of authority. Quite simply, you want to be in that hub, at the centre if possible (however unlikely), but at least in it. I like to think of this one as a good thing to remember in the future as search engines get even better at determining topical relevancy of pages, but I have never actually seen any granular ranking benefit (for the page in question) from linking out.
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