Subtitles, or secondary titles, allow you to show more information about your blog posts, which encourages visitors to read more.
Many popular blogs use subtitles in this way, including Medium, Buzzfeed, Mashable, Brain Pickings, and more.
Subtitles give you a chance to explain your title and tell readers what they will get from reading your WordPress blog post.
They will also encourage visitors to stay on your site for longer and read more content, which can increase pageviews and reduce the bounce rate. This sends positive signals to the search engines, so it may even improve your WordPress SEO.