Why do you warm a Dutch oven up before baking bread?
The point is you are actually using the dutch oven as the oven itself here--so you warm the dutch oven for the same reason that you would pre-heat your conventional oven: you want the bread to immediately go into the hot oven, instead of slowly raising the temperature. 

With a hot dutch oven, you get the "oven spring" that you would get from putting the loaf into a hot oven, and the dutch oven traps the steam that the bread gives off, so it helps keep the crust soft while the bread expands.  This is a great way to bake bread at home in ovens that don't have steam injection.