This would presumably let x86 windows games run on ARM hardware.
This is almost certainly meant for the next Valve VR headset, but ARM has so much better power efficiency than x86 that a future ARM based Deck would be a huge improvement to battery life.
Rosetta is for the game makers, proton is for the fans. So its easier for people to make the games work vs waiting on the game developers to manually port it using rosetta
This is basically what Apple's game porting toolkit does, except that's not meant for distributing to end users (probably because they don't want to expose their users to the bugs inherent to such emulation layers).