First, SEO stands for Search Engine Optimization. The goal is to help your website rank in the top 3 of Google for your main keywords.
Now imagine you need to find a plumber urgently, you go to Google and type “plumber in Hong Kong” or “plumber near me”, Google will show you 10 websites on the first page. Those 10 websites were chosen by Google because they are very well optimized for SEO!
As a result, these websites receive the vast majority of views from potential customers. If your website is not SEO friendly, then Google will never show it on its 1st page.