What is the main purpose of using keyword in SEO?

[michaelclarke]

What is the main purpose of using keyword in SEO.

[addisoncave]

Main purpose of using keywords to optimize the full site with a simple word that word referred as Keyword.Because it is quite difficult to remember all sites which are provide that facility. means if we have to check dresses then it is difficult to remember all sites which are related to that keyword. So we type dress on the search engine then the Search engine returns the result related to that products.