Even if you are using WordPress, you will often need HTML for posts, pages, widgets, etc.
ALL websites, regardless of how they are coded or what tools are used, are eventually translated to an HTML page. Regardless of which framework or programming language you use to build your website, it will eventually have to create an HTML page to show up in someone's browser. So, HTML is crucial.
The more you understand HTML, the better web developer you will be.
Even if you have no intentions of using HTML to develop your web pages or posts, being able to read and understand HTML will help you figure out what's going wrong when you encounter a problem.
HTML is the basic building block of the web. It's easy to learn and will probably save you several headaches and quite a bit of money in the long run.
You can find tutorials here
If you need help or have questions, just let me know here!