Search engines will index your site sooner or later without needing SEO, and you can see this through your website's logs, which reveals unknown search engines visiting because they know about your site either through the records in your domain registry or following certain patterns from free hosts/subdomains services if you have you have not a fully qualified domain name.
However, SEO is important to get your site indexed on top of other results and therefore a potential visitor can see it at first glance.