Why must you always preheat an oven when cooking?

Most cooking times are worked out for a preheated oven. This is because different ovens may have different heating up periods because of varying insulation or element sizes. For recipes to be consistent for any possible oven, they cannot take all the possible variations for preheating into account, and therefore require that the oven is already at temperature before cooking starts.

When you are familiar with how long your oven takes to heat up you can experiment with the oven not fully heated before using, but note that this will not work with some recipes that require a quick change of heat.

If not preheating, set the timer for at least an extra 2 minutes. Ovens vary in cooking time even after preheating.

Check the recipe before cooking. Sometimes preheating is, and sometimes preheating is not, necessary.

If you need to cook something in the oven for a specific amount of time at a specific temperature, then it must be cooked at that temperature the WHOLE time. If it takes 10 minutes to preheat it to 300 degrees, and you put the food in WITHOUT preheating, you undercook it, and if you out the food in WHILE preheating, you end up overcooking it.