Another reason for doing it is that the bran contains some fat. When you are washing the rice, one technique is to push down with your hand to cause the rice g

Another reason for doing it is that the bran contains some fat. When you are washing the rice, one technique is to push down with your hand to cause the rice grains to rub against one another. This polishes the rice a little bit more and removes a bit more bran from the outside (including any potentially rancid fat). Unless you have ridiculously polished rice, there will always be a bit of bran on the outside, so it can potentially improve the flavour. I eat a lot of rice on its own (no sauce, etc) and I do find that washing it this way improves the flavour slightly.