If a website is a body then the content of the site is surely the heart & circulatory system.
Well written, keyword focused (without spamming), informative and concise page content is as an integral part of optimising a site's performance as any other measure you can take.
Anyone reading your site is almost guaranteed to have arrived there by searching a keyword or phrase and it is vital that there is naturalistic and worthwhile content around that phrase on your site, not only for the sake of website optimisation, but to reach the right people with the right information at the right time.
A commonly made mistake is to stuff pages with keywords in the hope of improving search performance. While it is tempting to try to encapsulate any message you're trying to convey by using as many keywords as possible that approach is ultimately detrimental to long term website optimisation. Additionally, the end result is often terrible to read and ignores the fact you are writing for people rather than search engines alone.
SEO is a long term, many faceted means of improving your website and SEO content writing is one aspect of that which should not be underestimated.