You may think you know how your consumers are using your website, but you can never really know for sure without having some sort of tracking on your website. Adding Google Analytics lets you see how different marketing initiatives, or external factors affect your website. Using "GA" as your source of truth for digital marketing lets you see how consumers are getting to your website. maybe you have a banner ad on another website, and that ad tells you it has a lot of clicks, but with google analytics, you can see if those clicks are actually quality potential customers, or spam clicks.
Google Analytics can also be a great source to measure tests on your site. Simply changing the text or color on a button can have positive or negative effects on your bottom line. Using GA as a way to test the results of a test are a great way to ensure your website is always optimized to perform the best that it can.
Being able to track how consumers navigate your site is so crucial to website optimization. Using Google Analytics to see what page, or pages, consumers are falling off of the most, is a great way to optimize your website to be the most effective, and most user friendly.