Preheating the oven is something we all do without thinking, but this step can be eliminated in a lot of cases, saving you a bit of time and energy. Photo by Nita Lind.
As Epicurious explains, preheating the oven before putting the food in is only necessary with foods that need to be hit with a certain level of heat right off the bat to get 'em going:
Take bread, for instance: What the heat of the oven does here is provide the last little push the yeast needs to raise the dough to the loftiest heights possible before the flour structure sets; starting from a cold oven would produce a flat, dry, chewy product. A similar principle holds true for doughs or batters that have been leavened with baking soda and/or baking powder -- like those for cakes and cookies -- which also need a certain level of heat, applied quickly, in order to do their thing.
Basically, if it contains flour and/or eggs, preheat your oven. If it doesn't, you're free to pop whatever it is you're making in there as soon as you fire that baby up. Casseroles, roasts and sheet pans of vegetables all fall into this category so feel free to throw them in whenever you're ready, whether the oven is fully heated or not.
Do I Really Need to Preheat the Oven? [Epicurious]