Both Internal links & external links are important in link building technique.
External link - take readers from your website to another website, So it gives power in terms of ranking & traffic.
Internal link - take readers within your website, So it gives power to that external link in terms of credibility, bounce rate, link juice & moreover it looks natural to the eyes of Google.
Example :
100 external link & 0 internal links - Good in ranking but that Looks unnatural to eyes of Google - So, its a Spam!
100 external link & 30 internal links - brings additional power to the external links which always get you good ranking in very natural way.
Thus, the website has do-follow / no-follow links & internal / external links - That's best link building practice.