Headings are essential for screen readers and other apps. They are used to break-up content and make finding information easier. Screen readers cannot infer meaning from just formatting (such as bold or underlined text). When arranged in a logical order, styles/headings create a structure that screen readers can quickly access.
Outlook 2016 contains the same familiar “Format Text” menu in the Ribbon Bar for composing email. Selecting the pre-defined styles to distinguish body content, headings, etc., will ensure that the underlying HTML contains semantic structure and not merely visual style.