Search engines use the headings within each page to index the structure and content of your web pages.
Users often skim a page by its headings, and screen reader software usually generates a navigable list for each page, using its headings. It is therefore very important to use meaningful headings to show the page structure.
Some sites will have hidden section headings. If a design excludes meaningful headings, we will routinely add in “hidden” headings as we build each new site. These hidden headings are then used by screen reader software, to generate its list for the blind user.
An <h1> heading is only used for each page’s main heading. Every major section of a page should be labelled by <h2> headings, then the less important <h3>, and so on as necessary, potentially all the way down to <h6>.
Headings are invariably relatively sized – to generate visual cues for their relative hierarchy. They are not a styling tool!
If you want some prominent text in the middle of your editable content, using a heading to achieve this will BREAK ACCESSIBILITY on that page. You can toggle your WYSIWYG field editor into ‘HTML’ mode and then add style attributes, like style="font-size: 2em" and that will not affect the accessibility in any way: styling adds no semantic meaning to the mark-up and is only for sighted users anyway.