In short, a website which hasn’t had any SEO done on it isn’t reaching anywhere near its full potential.
Websites can be found by plenty of different means. By using pay per click (PPC), you can attract people to your site by buying up ad space above or below the organic results, however, it will cost you a fee every time your advert is followed.
You can promote your site on social media, but it is something which needs to be used frequently to see any steady traffic and conversions are typically very low.
Other websites can point visitors in your direction by using links, whether they via be something as simple as a business directory listing or from a news story. Again though, it’s an unreliable source of customers (although backlinks are a vital part of SEO overall).
SEO is about understanding user intent, and if done well, can consistently put your website in front of people who are searching for exactly what your company is offering.