Because that traffic is directly related to the quality of the backlinks your website has, the more authoritative websites that link to you, the better rankings and traffic you’ll get.
And of course you want to keep an eye on your Google rankings. When crawling the web, Google looks in particular for your website’s backlinks, to understand how your pages are connected to one other and in what ways. Certainly there are hundreds of ranking factors. But backlinks represent the most important metric for SEO.