Any time you're working on a multi-page project, you will have items that get used across multiple pages. Navigation is the most common. If you're copy+pasting

Any time you're working on a multi-page project, you will have items that get used across multiple pages. Navigation is the most common. If you're copy+pasting anything every time you add a new page, you're wasting time… For me, master pages and something like Illustrator Symbols ("master elements", "template elements", whatever) save vast amounts of time for complex multi-page projects.