That's not entirely correct. The reason Nvidia and AMD can release a single installer for all their video cards is because the installer auto-detects the card and enables/disables the features which are/aren't supported by that card as part of the install process. If a feature is not supported by the video card hardware, the drivers are usually set to emulate it in software, rather than error out and fail when a program tries to use that feature.
When you upgrade video cards, in rare cases the new driver install can fail to change some of your old driver settings. If you upgraded to a newer, better card and this happens, it's possible for the driver to still be using software emulation for a feature that your new card supports in hardware. The result being that your games all run, the new card is faster than the old card, and everything seems to be working as they should, but you're not getting the full performance that the new card is capable of.
In theory simply reinstalling the drivers after you upgrade video cards should enable all the correct driver settings. But Windows being the huge, complicated thing it is, occasionally things go wrong (more frequently if you cross-upgrade between AMD and Nvidia). And this is a silent, undetectable failure - you don't get any error messages or indications that cruft from your previous driver install is still hanging around. The only symptom is you're not getting the full performance the new card is capable of, which you can't detect because by definition you've never used the new card before in your system. So DDU is a good idea after a GPU upgrade for peace of mind that your drivers are in fact set up correctly for your new card.
I would set a System Restore point (Windows 10 disables System Restore by default), download the latest driver installer for your new GPU, disconnect the computer from the Internet (so Windows doesn't try to auto-install drivers), run DDU, then install the GPU drivers again. Just for peace of mind. It's easier than running a gazillion benchmarks trying to figure out if your new GPU is running as fast as it should be.