If you plan to look at moving things, like scrolling, which is a central part of using the device, 120fps makes the scrolling less blurry and therefore more pleasant to look at.
This is like asking why people have a hardon for cars with nicer materials for seats and interiors if a normal cheap plastic interior could pass all the same crash tests.
I can see the 120hz nice scroll when I’m paying attention but I’m normal use on these devices I forget about it. Basically it wouldn’t be a dealbreaker for me but I agree it should probably be the standard
The only genuine performance increase I could see is if you're using it for drawing/notes because the Apple Pencil samples at 240/s so you would see a difference in drawing smoothness between 60/120/240hz screens. This would mirror the use case in fast-paced and high action games - more info for your eyes sooner allowing better reaction times.
Also, isn't ProMotion just branded VRR? Allowing for lower battery use in static/low change applications?
It’s a penis measuring contest for display enthusiasts, much like gold connectors are for audio enthusiasts.