when do you actually need ddu before a driver update?

Note that DDU wipes your driver settings too. If you've got a custom profile in the nvidia or amd control panel, screenshot it before.

Almost never for a normal update. Install the new driver over the old one and you're fine. Use DDU when you switch GPU brands (nvidia to amd or the other way), when a driver install is failing or leaving you with crashes and black screens that the old driver didn't have, or when you're rolling back a long way. Run it in safe mode and pull the network cable first so Windows doesn't sneak its own driver in before you install yours.