There are two reasons. One is the give the remaining fruit a better chance of actually getting ripe. With a large section of the fruit gone, the vine will give more energy to the remaining fruit, giving it a better chance of getting ripe. It is going to be a very late harvest this year--around October 26th for Pinot Noir, and maybe November for Riesling. It's going to be a nail-biter at the end of October to see whether the rainy season holds off just long enough for the fruit to ripen. 
At Coeur de Terre they have cut the fruit back to one cluster per shoot. Many vineyards use two clusters per shoot, and some leave two clusters on one shoot, and only one on the next. This year is the most austere I have ever seen. Can you imagine what it must feel like to nurture these plants, to care for them and prune them, to feed them and hope they grow--and then go and throw half of them on the ground? I'm not sure I could do it. You can see how beautiful some of the fruit is at the photo to the right (complete with yellow jacket!) The second reason to drop fruit is to increase the complexity and depth of the fruit.
I will be starting my internship with Scott in just a few weeks (or whenever the grapes to finally get ripe.) I'm excited to get to experience crush, and see how wine is actually made. It will make me appreciate all that much more the beautiful wines we are able to enjoy here in Oregon. Wine is an amazing symbol of God's grace to humanity, of how much God loves us. That grace, however, does not come cheap. It requires an incredible amount of hard work and dedication. I think I'll have a much more concrete picture of that work about a month from now.