Internal Linking is a very important SEO component, search engines like Google doesn’t rank the whole site, but each individual page. While for the most website the home page will have the most link and traffic. For this reason search engines, check all links on each page and index them, this is how internal linking is doing its job.
Linking between websites is an important factor, you can read more here about Link Building. Getting external link for your website will increase its value and rating for search engines. Talking about internal links, they are also important and can also add a good part of SEO performance to the site. When you add a link on your homepage for example, it will automatically tell to the search engine that the link can be trusted and its from the same category. This really helps search engines to index and build a structure of your site.
Internal linking is important from visitors perspective. Search engines will scan and rank each page of your site, and will see each page as a potential landing page for your visitors, when they open it from a link or especially from a search engine. Also internal linking helps to move traffic through your site, so older posts will still be active and productive for the site. Having internal links, visitors will also be pleased to see the next step that they can do or read to understand mode. This will not allow users to run from your site to a search engine and search for more explanations on the data you posted.
Linking between posts will always keep reader engaged for longer time as they jump from one post to another to find more information about the topic they are interested. So, you must spend more time to build a good internal linking system between articles on your website which will give as result, more targeted traffic and will increase SEO performance and rating.