SEO copywriting is about high-quality content
Professional SEO copywriters don't write for robots, we write for results
It's keywords that people search by and content they're looking for. Successful SEO is all about delivering high quality, relevant content written for your audience in a way that's optimized for search engines.
Too often clean, crisp copy is destroyed by keyword stuffing and writing for search engines, losing all effectiveness for real visitors. Text ceases to flow and individual style becomes sterile and impersonal — written for robots.
It's no secret that high-quality content is the lifeblood of any website (just ask Google) — it's what people search for and what convinces them to buy. Without engaging web copy SEO only delivers high bounce rates, and when times are tough, you don't need visitors, you need customers.
Google gives the following advice for optimizing your website:
Create a useful, information-rich site, and write pages that clearly and accurately describe your content […] Think about the words users would type to find your pages […] Make pages primarily for users, not for search engines. — Google
And Wikipedia defines SEO like this:
Optimizing a website primarily involves editing its content and HTML coding to both increase its relevance to specific keywords and to remove barriers to the indexing activities of search engines. — Wikipedia
This boils down to the advice that all good SEO professionals will give — "Write high-quality content for real visitors, relevant to what they're searching for in a way that's optimized for search engines to index".
This poorly kept secret shouted from the rooftops by Google is why SEO copywriting is fundamental to successful Organic SEO.