Google has some Webmaster Guidelines. According to these guidelines search engine decides if it is black hat SEO or not. Black hat SEO users never think of user experience. They only need high rank on SERP and this high rank leads to more traffic.
Without a search index it is hard to display search results within seconds. Incase of not having search index, search engines (Google, Yahoo, Bing, Internet Explorer etc.) take up resources directly from the server and make the site slower.
Checking keyword difficulties for content writing is so much important. We have to choose low competitive keywords or low difficult keywords , so that there may be a possibility to rank on SERP .