Have you ever come across a website that read in a boring or mundane way? Many people try to "stuff" their pages with keywords in an effort to rank higher for their keywords and phrases, which is actually worse than not using the keywords enough. A qualified SEO content writer can write interesting, valuable content that makes sense while still using the keywords in a way that isn't obvious to the reader. In my opinion, a keyword density of about 1 1/2 to 2% is plenty, and the content reads in a natural, flowing manner.
When trying to choose a great SEO content writer, ask for samples of their work. Links to websites, articles, blog posts or other content they have written will give you a good idea of their abilities when it comes to good content and search engine optimization. The internet revolves around content - and you want yours to be the best it can possibly be!