The simple answer is that hertz determines how many frames you can actually see. A PC that outputs 100 FPS with a 60 Hertz monitor will still be limited to 60 FPS on its screen. More FPS displays a faster image, but you are still constrained to the refresh rate of your monitor. That said, 100 FPS on a 60 Hertz monitor will still look better than 60 FPS on a 60 Hertz monitor.