A screen reader user navigates through headings to understand the content of a new page. A well organized document aids the user to orientate himself to the specific section he needs. Headings and sub-headings help to categorize and organize content in a way that the flow of perception is intuitive.
Headings are already included in the Styles libraries of Microsoft Office Suite applications such as Word. Rather than styling words to make them look like headings by increasing the font, using bold, italics, caps, or any other technique, it is easier to click on the appropriate heading style. Headings from the styles menu contribute to consistent design. These are also replicated in the navigation pane for easy access in long documents.
In html, resorting to h1 through h6, does the job.