Why do you need to preheat the oven before baking?

Check the recipe before cooking. Sometimes preheating is, and sometimes 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.