First of all, humans can see flickering at 30 Hz and even at 60 Hz (there are different zones in retina with different "specs" some see color, some see motion).

First of all, humans can see flickering at 30 Hz and even at 60 Hz (there are different zones in retina with different "specs" some see color, some see motion). But even if it was not visible there is still a huge benefit of higher framerates: the latency between input and output. When you press a button the time to update the screen to reflect your press is 1.5x frame time on average at best (wait till the current frame ends, which will be 0.5 time on average and render the next frame with the input). In practice the output is buffered in many places and the latency can be as long as 10 frames. With 60 Hz that will give you ~166ms latency, which anyone can feel.