You finally finish that 30-page document, and then the reviewer says, “It’s perfect! Can you make one little change?” If that “little change” is the color of all your topic headings, and you did these manually, you’d need to go through your document and change each one. Ouch!
If, instead, you applied a heading style from the Styles Gallery (Home tab), you could just make your changes to one heading, then right-click the gallery entry for that heading style and Update Heading # Style to Match Selection. Magically, all your headings would change to the new format in minutes, not hours.