Sugar in the dough can provide additional food for the yeast to feed upon, this is especially true if you are planning to give the dough a very long fermentation time. It also contributes to the crust color development in the oven. This can be a double edged dagger though. While the sugar can help the dough color up in the oven, it can also lead to excessive charring/burning of the crust, especially if you are baking at high temperatures. The shorter baking time brought about by the faster crust color development with sugar present, can reduce the crispiness of the finished pizza, and lastly, depending upon the type of sugar used, it may contribute to the finished flavor profile of the baked crust (think sweet, or malty as when nondiastatic malt syrup is used).
Tom Lehmann/The Dough Doctor