It's not a trivial feature. The Class A surface on an iPad is the screen.
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.
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