IIRC it's needed for recording, streaming (both game streaming and outputting to Twitch), fps overlays, Ansel, and Filters.
All Experience adds to Ansel is "easier sharing" of the screenshots: https://developer.nvidia.com/Ansel
MSI Afterburner is another one I've seen mentioned but I've not tested it.
(https://moonlight-stream.org/ is a fantastic open-source implementation of the client side part)