My guess are people who want to PC game but don't want to deal with building a PC themselves - there's a decent market of pre-built gaming PCs that this would be competitive with.
https://www.newegg.com/Gaming-Desktop-PC/SubCategory/ID-3742
https://www.newegg.com/Gaming-Desktop-PC/SubCategory/ID-3742
That said, I've never had a Steam Deck or tried to seriously game on Linux, so I may be out of touch with how much smoother the picture is in an all-Proton world.
(the laptop issue turned out to be something in the firmware asserting BC PROCHOT for some reason; for now we can periodically clear it with the ThrottleStop utility, but who knows what the actual underlying problem is)
valheim started with extremely poor steam deck performance, but at some point, the team did steam deck optimizations that got it humming nicely enough