Why link building is important for SEO is because without links there would be no recommendations going to your website . The main reason of the Internet is to exchange information that is primarily the main concern of the Internet. Google is a big player and how the Internet processes information so if you were to build links from different websites going to your website you’re primarily telling Google and the Internet that your website has content that can be used and for many purposes.
Links and the quality of links cannot be underestimated because they are not just letters or a mechanism for the web browser to skip from one page to another– links are telling the Internet and the Google bot crawlers that the page being linked to has more information on a certain topic.
These kind of legitimate links that link from one page to the other on relevant topics can boost the presence of the page.
Collectively linking from different domains to the page on relevant topics is going to further heighten the presence of that webpage.
This is the main reason that if you have a website that you do not have certified links from different pages coming to your domain or your page: the Google bots that your page has Relevant information on a certain topic you cannot get ranked.
We need to use this to our advantage instead of being overwhelmed by thinking of how many links — you should not be thinking in business terms of goals but think about how much information and what kind of quality information you can contribute is the winning combination.
Resources : Why SEO is your best Friend?