Crawling is the first step for SEO which defines the knowing of which pages can be found on the websites. All of the websites are not saved anywhere which can be used as central of websites. Since that, Google and the other search engines have to crawl around the websites and add them next to the most known ones. The search engines are aware of which websites you are using and they crawl from there to similar pages and explore them to add to the system. Crawling makes Google suggest to you the websites you could be interested in based on your previous visits.