That doesn't answer the question of why anyone would run the Windows version of Steam on Wine, rather than running the Linux version of Steam, which would use Proton (which is built on Wine) automatically and painlessly.
I don’t know, I don’t game under Linux. But from my own wine experience, maybe to play some games that were ported to Linux and available first-party but have some form of crippled experience or bugs and playing the native Windows port is the way around it.
(Eg Netflix plays on Firefox/Chrome under Linux, but if you run Chrome under Wine on Linux instead, you get 1080p video instead of crippled 720p crap.)
Plenty of older games only seem to work with certain versions of Proton/Wine, DXVK etc. There are projects like Bottles which let you manage multiple Proton distributions https://usebottles.com/
That's true but in Steam on Linux there's a dropdown menu in the settings that lets you choose the Proton Version. And tools like protontricks can add additional wine/wine-staging/proton versions that would show up in Steam.
That doesn't answer the question of why anyone would run the Windows version of Steam on Wine, rather than running the Linux version of Steam, which would use Proton (which is built on Wine) automatically and painlessly.