News Courtesy of Yoast.com:
Well-written copy improves UX
At the risk of stating the obvious: unintelligible copy makes for bad user experience. Nobody likes to read something that’s incomprehensible, boring or stuffed with keywords. All your web copy should, therefore, be aimed and focused on your audience, giving them the information they need, a solution to their problem, or an answer to their question.
First and foremost, you should write great copy for your visitors. This fits in nicely with our vision of holistic SEO. If you write text people don’t understand, you won’t help people find what they need. What’s more, you’ll end up with unsatisfied visitors, that’ll probably bounce back to Google when they hit your site. Google analyses user signals like that to determine how visitors experience your site.
Ryan’s Take 
Back in my early days of web design & SEO with Yoast, I didn’t really pay much attention to the readability score. After all, if I could understand the targeted posts and pages, surely others would too. However, googlebot has evolved to a point where it really tries to emulate a live person. You can’t expect to trick it by placing a keyword over and over again without any rational explanation. As Yoast points out above, the user experience is an important metric in rankings these days.
If visitors to your website continue to search for queries that aren’t satisfied by your website, it’ll hurt your rankings. On the flipside, if visitor’s don’t return to the search results, your site will be rewarded with an increase in rankings. This means that you need to keep your audience engaged with great information that is quickly available and makes sense. Unless you’re writing a technical post, try to avoid big words that might be unfamiliar to others.
The full write-up that the Yoast team provided also expands upon voice search and its growing popularity. When people use their preferred voice assistant, most of the time questions are simple and to the point. Google’s goal is to present the answer to those queries in a similar manner.
If you’re already using the Yoast plugin for WordPress, I’m sure you’ve taken the necessary steps to get a good score for SEO. That same effort should also be applied to the readability score. In my opinion, it is a lot more difficult to get readibility “in the green” than the SEO score. However, doing so will not only make you a better writer, but it might also help you outrank more popular competitors with a poorly written topic.
