Headlines and subheadings highlight key elements of the topic, making it easy for readers to skim and leading them through the content.
Search engines can identify content by using keywords in subheadings. This allows search engines to inform the reader about the article’s key points quickly.
However, this does not mean that keywords should be forced into subheadings. Keywords should make sense and sound natural.
The title tag hierarchy should be used for subheadings. This is why you should use smaller heading formats.
You don’t need H2s for most articles. However, if you have to separate H2s with lots of content, H3 can be used.
Use header tags as directed by this SEJ article on best practices.