A signed contract you don't want forwarded
You've just sent a draft contract to a prospective client. It's not the final version, and you don't want it ending up in someone else's inbox without context. You add a diagonal "DRAFT — Not for distribution" watermark across every page in tiled mode at 30% opacity.
You drop the PDF into the tool, type the text, set tile mode on, and pick a soft grey at 30% opacity. The preview shows the diagonal pattern across the first page exactly the way you wanted it. One click and you've got a watermarked copy ready to send — and the original PDF stays untouched.
The next time the same client asks for a revision, you just reload your PDF, the watermark is gone in the new version, and you send the cleanly branded "Final" version instead.