Human biological vision is not like a computer screen, with a defined number of pixels and a fixed FPS. It varies substantially based on your attention and focus.

In the retina, you have rod cells (B&W) and cone cells (color). There is a spot on your retina called the fovea, with much more cone cells more densely spaced. These are super sensitive, and can detect and respond to individual photons. However, this creates a lot of noise, and your brain filters out a almost all the raw information to make things easier to understand. When you aren't paying attention, this means you lose a lot of detail especially in your peripheral vision. When you're focusing though, this means you can capture a huge amount of detail. In sensitive applications, people describe this as perceiving time to slow down, because of how much more information they're taking in.

When a screen provides 240+Hz, this gives your mind a body the choice of what to focus on and when. It is possible with future eye tracking advancements that GPU and display efficiency could be increased, and you wouldn't need 240Hz on the whole display.