The target while writing general web content is that it should be able to be understood by 13-year-olds and above. Special cases, like scholarly articles aside, the internet is intended for viewers of almost all ages. This means that the content on it should be understood by all too.
If your content is too hard to read, your viewership will decrease. It has become more and more important to become user-oriented to rank higher in search results. So, receiving a high readability score will help you a lot.