Garvey hit the nail on the head.
While many of us here, at this web site don't let the pizza skin set around for any length of time between dressing and baking, that isn't always the case in a commercial pizzeria establishment. In order to cope with getting "slammed" at say, 7:00 p.m. of Friday nights (for example) it is a common practice to pre-sauce the pizza skins and hold them in the cooler until needed. When they get slammed, all they need to do is to pull a pre-sauced skin and add the toppings. This helps to keep the delivery time between an order being place, and the pizza being delivered to the customer's table more reasonable. Oiling the pizza skin prior to sauce application helps by creating a moisture barrier, thus preventing/reducing moisture migration into the dough prior to baking. When using fresh tomato slices instead of a sauce, the oil application again reduces the moisture migration into the dough as the fresh tomato slices begin to release their moisture during the baking process. If blended with garlic or other herbs, it will also add another dimension of flavor to the finished pizza. Just don't get carried away with the oil, if you can see a reflection (shine) on the dough from the oil, you have added all that is necessary, if you add too much, you can create a situation where the toppings just slide off of the slice with the first bite. Take and bake pizzas also benefit from the oil addition too as it may be hours, or even days between dressing the pizza skin and baking it. If you want to see what we are working against here, just put a spoon full of your sauce on a china plate, and cover it to prevent evaporation, then come back to it in 30-minutes, or so and you will typically see a ring of water around the sauce, this is the water that can soak into the dough resulting is the dreaded gum line just beneath the sauce layer.
Tom Lehmann/The Dough Doctor