You need to get your website index by search engines so people can find you.
If you were to think of your site as a shop on the main street of your local town, people would walk past, see your displays and shop front and maybe pop in. This however is not true for the web. The way the internet is made there is no such thing as passing trade. You can have the best website in the internet, have a revolutionary product, have a very important message that everyone should read but no one will come if they do not know where it is.
To get people to visit your site, to read your message, to buy your products they need to know where to find you. People need to know your website address, your URL or be able to follow a link to your site.
How do we tell people about your site? Word of mouth? Yes, this works well with social media but SEO is crucial to the longterm success of your website.