Part of the answer is that the tiles increase the thermal mass of the oven, so that there's less of a loss of temperature when you open the oven. More consistent oven temperature means more consistent cooking, and in particular, sliding a pizza directly onto the hot tiles means a faster-cooked pizza with a better-cooked crust. (This still applies if you're putting the pizza on the tiles in a pizza pan.)
Even food that doesn't do directly on the tiles will cook more evenly because the temperature is more even. This is why many large commercial ovens are brick-lined. (See also tandoor (http://en.wikipedia.org/wiki/Tandoor).)
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Even food that doesn't do directly on the tiles will cook more evenly because the temperature is more even. This is why many large commercial ovens are brick-lined. (See also tandoor (http://en.wikipedia.org/wiki/Tandoor).)