I recently wrote about the need for the website owner to take responsibility for SEO and not rely 100% on a theme or a plugin. Themes and plugins do of course have some benefits if you understand what it is they do, and what they cannot do.
The benefit of themes and plugins
‘SEO-friendly’ themes and SEO plugins present your content in such a way that it is easy for search engines to understand, determine the relevancy of, and navigate. So if you’ve targeted your content for specific keywords, the search engine will have no doubt what that piece of content is all about. The content and keyword research is up to you. No theme or plugin alone can make your site rank highly if your content is not valuable and targeted. Also, this all relates to on-site SEO, but ranking is also determined by off-site factors such as incoming links from other websites. Themes and plugins cannot help you with these off-site factors.