Content writer and SEO writer are both different in terms of the goal and job description. Although both tasks require writing which is based on the standards given by the editor, the output may not be the used for the same purpose.
Content writers are responsible in preparing written materials for a certain website, course, or products. They’re focused on what to write or include in the page, rather than external factors such as ranking, or success of an article once released.
Meanwhile, SEO writers are concerned with keywords or phrases for a specific goal. Most of the time, they’re hired to increase the organic traffic of a website.
You can work as a content writer but not be concerned with other factors. You may also work as a SEO specialist but not be concerned with writing or output.
Or, you can work both as a content writer and SEO writer. This is called SEO content writer.