The belief here is that the keywords you use in H2-H6 subheadings are more heavily weighted in Google’s algorithm than words in plain text and that tags are therefore a ranking factor of their own.

Search Engine Land » SEO, PPC & AIO Guides » The most important HTML tags to use for SEO success Share HTML tags are the backbone of SEO that’s overlooked by many digital marketing professionals.

Google's John Mueller said that the size of your header tags, i.e. H1s, H2s, etc, does not matter for SEO or ranking purposes. John said it might matter for users and conversions but for SEO, nope.