First is simply the convenience of having it all built in to the gaming platform you're using instead of juggling other software. Plus Steam will host content you want to share for you, which neither AMD nor Nvidia does. Also, neither AMD nor Nvidia's offering providers a two hour rolling recording that you can just skim through and pick clips from at your leisure.
Second is the hope for better reliability. Shadowplay/replay/whatever nvidia is calling it now just stops working at random for a lot of people, myself included, with no warning or indication until you hit the "save replay" button and get a popup telling you that its not running. I also wrestled for a while with it recording the wrong screen when I had two monitors, so I'd just get clips of my second monitor desktop with the audio of the game. There are lots of people hoping that Steam will manage better.
I've seen initial reports that at least on Steam Deck it's far less impactful than any other recording solution available so far (decky recorder/obs/whatever).
Like you though I'm interested to see a detailed look at how it does on a standard Windows gaming PC though, where you've already got well established low impact solutions like Shadowplay or Relive.
Many are, yes. But for some like me who play via cloud streaming where at least Nvidia stops that function, it's a great thing. It's also nice to have it all in one place, most people already use Steams screenshot function so having this too is just nice. This has been a commonly requested feature for a long time, it's only weird that it has taken so long for it to exist.
No but I fucking hate geforce experience and the fact that I need to have an Nvidia account to use the features of my hardware.
Now I can remove that garbage app from my pc, thanks Valve.