When a potential customer searches for your product/ service. Your website won’t magically appear for them. Your site has to be worthy enough to search engines (Like Google).
This means making search engines happy, nearly every part of your website, what you can see and what you don’t see, all prove to Google whether your site is worthy of being shown to the customers.
Wonder why you aren’t getting customers from your website, chances are it isn’t optimised for search engines. Hence the term “Search Engine Optimisation.