Define The Main Purpose Of Using A Keyword In Seo?

Define the main purpose of using a keyword in SEO?

 

The main purpose of using the keyword is to optimize a website for better rankings on search engines. It is very difficult to remember all websites which provide that facility. It means that if I check domains then it is very difficult to remember all websites which are related to that keyword. So I type domain keyword on the search engine and google search engine returns the result related to that product.