Why is it important to preheat cooking equipment before cooking?

You should always preheat equipment such as ovens and fryers so that they are cooking at the correct temperature when the food is placed in it. This will ensure proper even cooking of the food.

If you are using an electric stove, you may want to preheat your pans because it takes so long for electric heating elements to come to temperature, compared to gas burners. If your recipe says to cooks at medium for 15 minutes, you won't be at "medium" on an electric stove for several of those 15 minutes unless you preheat.