Jerry, if one were to form a pizza, put it on a room temp stone and then put the stone in the oven, the poor conductivity would cause it to take forever to cook. Preheating the stone, though, changes the playing field. Instead of the distant (ie inefficient) bottom electric element or gas flame baking the bottom of the pizza (through the middleman of the metal pan), the heat source is the stone. Well, that is, if it's a good thick stone that can store a lot of heat.
There's a lot of tricks to shortening baking times with pans (such as using black colored, heat absorbing pans, putting the pans as close to the heat source as possible and/or working with a red hot/maximum radiation element), but, unless you have a freakishly high watt/btu bottom burner, you'll never hit the kind of short baking times you'll see with a good stone. And short baking times are the key to great oven spring/great pizza.