When you open your eyes, your brain must automatically process a tremendous amount of info: the bandwidth of the 2-D moving image on your 2 retinas is huge! Not only that, the image is hard-wired to innumerable automatic reactions in your ears, emotional centers, etc. IOW, when you open your eyes, you really give your brain a tremendous amount of work, whether you know it or not (if you see a lion charging at you, you will turn white and start running before you realize what you are doing, or if something is thrown at you, you will blink automatically; try walking or surfing with eyes closed). When you close the eyes, all this brain resource is immediately released, available for anything else. Thus it becomes easier to concentrate on accuracy and musicality. At the same time, the audience thinks that anyone who can play with eyes closed is a genius, whereas in reality, it is easier (once you get used to it). Therefore, when you really need to concentrate, close the eyes; however, make it a point to practice with eyes open and looking around (and even talking at the same time) so that you don't develop a lazy brain that cannot play piano and process other info at the same time. In piano practice, the most important job is to increase your brain energy and stamina -- closing your eyes will not help you accomplish this.