MORE FOCUS KEYWORDS
With the free version you can use one keyword per post, but often a post ranks for multiple keywords. This you can achieve by upgrading to Yoast premium.
ADVANCED KEYWORD OPTIMISATION
When writing or editing a blog post you will get more premium tips on how you can optimise the blog post for the keyword.
A redirect manager is a must-have. It allows you to rename your blog posts urls, and easily create a bridge between the old url and the new url without loosing any SEO juice.