I think this is a matter of preference. When my mom was teaching me to cook, she taught me to wash the rice. At the restaurants, we never really washed the rice, and the rice came out fine. The concept behind it is to remove any impurities that might come from the processing of the grains, like pebbles, faulty grains, and excess starch. In recent years most rice is coming relatively clean, so I do not wash my rice all the time. I do, however, pick through my rice to eliminate any grains that might seem harder than the rest, ensuring there are no small pebbles or foreign particles.