Headings are necessary for creating an accessible Microsoft Word Document. They provide a simplified structure for organizing and browsing content. If the document is very long, headings can save readers time understanding content without reading through the entire document. Headings also help people who use screen readers navigate each section more effectively:
You may be tempted to create a heading by highlighting text and increasing the font size, however you should never use font size alone to create headings since screen readers do not detect font size. It's important to use Microsoft Word's built-in styling to structure headings:
You can also apply headings using keyboard shortcuts to assign headings:
After applying headings, the Navigation panel should open up. You can also access the Navigation panel by entering Ctrl + F on the keyboard.
Source: Microsoft Word: Creating Accessible Documents by WebAIM.