Search engines are smart, but they still need help. The major engines are always working to improve their technology to crawl the web more deeply and return better results to users. However, there is a limit to how search engines can operate. Whereas the right SEO can net you thousands of visitors and increased attention, the wrong moves can hide or bury your site deep in the search results where visibility is minimal.
In addition to making content available to search engines, SEO also helps boost rankings so that content will be placed where searchers will more readily find it. The Internet is becoming increasingly competitive, and those companies who perform SEO will have a decided advantage in visitors and customers.
Source: Why can't the search engines figure out my site without SEO?