Typography styles now have a default HTML heading level. This makes it easier for the heading level to be set correctly when you're writing content. The heading level can always be changed in the editor.
These can be important for SEO. The HTML heading level can give screen readers and search engines indications of how your content is structured and what it's about, but you may want to visually distinguish this from how those headings are styled. Now with these defaults, each heading can automatically be set to a Heading size you choose (1-6) to make it quicker to automatically set these across your documents.