It is recommended that oven is Pre-heateded before use for several reasons:
1. Pre-heating to 350 degrees before roasting helps close the pores of your protein. This helps seal the moisture in similar to the concept of searing.
2. Pre-heating a oven before baking drives the humidity out of the air in an oven. It helps lessen the effects of humidity on your cake or pastry. With identical portions of ingredients and a recipe followed through the letter. You would experience a difference in baking characteristics in February vs August. Pre-heating helps negate atmospheric influence on cooking time and humidity.
3. It helps things cook faster!
NOTE- If using a clay dish or pot, it is recommended you start in a cold oven.