To objectively compare the performance of different graphics cards, cpus, 3d settings, etc. Sure, it's easy to see huge differences with the eye, but smaller differences are hard to see, and sometimes a bunch of small improvements add up to a significant one. Plus, when describing game performance to someone else, 15fps is more objective than saying "it runs ok". To some people people, 15fps is "ok", to others, only 60fps or higher is "ok".
wasabipeas: Frame rate is speed at which the graphics is drawn on the screen. Usually frame rate is measured in frames per second, or fps.