SpokaneMan, we always did it to get a little browning (very little) on the upper side of the crust, so that it wouldn't be "doughy" underneath really wet toppings. we use tomatoes all the time, and found that it was all very slippery and doughy under the toppings. precooking gave us a little crispiness. that, and a thin wash of olive oil before putting the toppings on.