It is important because 1) It helps in SEO 2) It makes it easier for the users to navigate through your site
yes internal linking is good it help to navigate your site. it also help google to crawl all pages of your website.
Links are good - as long as they are not over done. Internal or external don't matter much. Typically I keep the internal links per page to no more than two per page / per 300 words.
This is an easy way to boost traffic to other pages in your website. Also, if you archive any of your content, link back to that as well when creating new content. Also, make your anchor text search engine friendly. The more relevant keywords that link to another relevant page, the likely that page is to get indexed and appear in the search results when a user is searching for that term.