If you are talking about Search Engine Optimization for your website then link building is probably one of the most important aspects of SEO process that will surely be discussed as a part of Search Engine Optimisation process.
As each and every link works like a vote to your website that adds value and popularity to your website, every link is very important and should be given importance if not! The internal links are like building blocks for your website that enhances your website popularity in the search engines.