I know (of/first hand) a bunch of musicians who buy the top of the line one every generation because they "need" it. For video work you do need as much raw power as possible but for just about anything else you can honestly get by with a Macbook air these days (especially given that a lot of the top of the line customers are probably using external inputs rather than software synths!)
Apple had a renaissance with the old pre-2013 mac pro, and mostly people used pcie to add graphics cards to their system. the 2013 mac pro sort of offhand killed that with its mediocrity.
But nowadays it seems like graphics cards would need non-trivial OS support, so who would put together pcie cards for macos?
It really doesn't offer any huge benefits over the Mac Studio.