Headings are vitally important when assessing document structure as well as scanning and navigating a webpage (see LevelAccess's simulation of Web Headings for Screen Readers). They also help with search engine optimization (SEO), letting search engines know what topics your webpage contains.

Headings do not stand alone. They introduce subject matter. Headings should not be used to make paragraphs stand out, as that makes the content unscannable by your constituents. They should be no longer than 140 characters.