Why should you pre-heat the oven?

The main reason is that when cooking from a recipe, or following the instructions on pre-packaged food, there is a recommended time that the food needs to be cooked at the right temperature in order to be cooked properly and safe to eat. If the oven hasn't reached that temperature yet, you may be cooking it for the wrong length of time.

There are lots of chemical reactions occurring when you cook food, and different reactions occur at different rates depending on the temperature. This means that while your food is in the oven below the right temperature, you will not get the right balance of reactions occurring while you cook. The reactions which give food it's distinctive 'cooked' flavours and colours are Maillard reactions (between sugars and amino acids) and caramelisation (the oxidation of sugars). These reactions work better at higher temperatures, and so your food will look and taste better if you wait for the oven to heat up.

Also, while the food is pre-heating it will be losing water by evaporation, meaning that the cooked meal is likely to be drier and less succulent.