Why preheat the oven?

[mariner2]

Why is that it always says "preheat the oven" then cook for 35 minutes - what's the difference if you just cook for 40 minutes?

[Clanad]

Ovens differ widly on how long they take to reach the recommended temperature. The recipes assume an operating tempearture throughout the recommended cooking time and therefore, taking into account the length of time to reach that temperature (and maintain it) can cause an absolute failure...

[pitapan]

*********FOLLOW THEESE INSTRUCTIONS********
put the oven on whatever and take it out whenever.

[lafrancaise]

If you put a cake mixture (or yorkshire puddings) into a cold oven, the mixture would dry slightly (as the oven was getting to the right temp.) before starting to cook and would spoil the cake. A roast would do the same however, no problem with a casserole.

[Gef]

Try it and see.

[mariner2]

That doesn't make sense - if you have to pre heat the oven for 5 or 10 minutes before putting a yorkshire pudding, cake mix or roast in, then they are still going to dry slightly sitting where they are, instead of being in the oven from cold aren't they?
Even if they go straight into a hot oven, the first 5 minutes must involve some drying I would've thought.

[rightproper]

As has been previously mentioned by not prewarming the oven anything placed into goes through transitional temperatures ie it warms the food up before actually cooking.
if you are roasting, the centre of the joint will be warmed before the browning of the outside making it hard to achieve rare.
In baking you want to achieve a skin so that escaping gasses will be trapped making it lighter and more delicate in texture.
It is also a health risk as the food will remain at optimum bacterial temperature (37) for longer.

[Gef]

The timings on pre-packed meals always make it clear that they are guidelines. The problem is that different ovens take different times to heat up.

right's answer is just nonsense.

[mariner2]

I thought right's answer was pretty good!
I didn't understand why you have to preheat ovens, but he's explained it