You're right, you can't see at 120 fps, because that's not how human vision works. You can't see at any fps. However the more fps there are, the sooner you get

You're right, you can't see at 120 fps, because that's not how human vision works. You can't see at any fps. However the more fps there are, the sooner you get the information, and so regardless of the lag on your end (the biology behind the eyes, the brain, and the connection between them), you still add some delay coming from your computer.