Why does it matter whether you put your biscuits in the oven and turn it on or preheat it and then put your biscuits in? Won’t the heating up stage give you a sort of head start?
The biggest problem is that foods being cooked with this method slowly warm with the oven, causing them to become done on the outside before the insided is done. This is especially true for cakes, cookies, and bisquits. By the time the middle is done, the outside is burned. And with cakes, instead of being flat when done, the center will rise. This can be a problem for layer cakes that are going to be frosted. You also get cakes with big air pockets instead of a uniform texture. My mother’s secret for her wonderful melt in your mouth bisquits was to preheat the oven warmer than required and then turn the oven down to the recommended temperature when the bisquits were put in the oven. For items that take a long time to cook, it is not a problem. As the packages always say, for best results, follow the directions. There is a valid reason when they say preheat the oven.
Or, for the short, and I would think obvious answer: For the X number of minutes that it takes the oven to warm up, the food is cooking at the wrong temperature. Converting a 30 minute baking time into the correct amount of time becomes a small calculus problem.
>> The biggest problem is that foods being cooked with this method slowly warm with the oven, causing them to become done on the outside before the insided is done. This is especially true for cakes, cookies, and bisquits. By the time the middle is done, the outside is burned.
Common sense and thermodynamics tell me this is the opposite of the truth. The higher the temperature the sooner the outside will develop a crust while the center is still not done. That is why you can fry icecream in scalding oil.
If you put the item in an oven which is not so hot, then it will cook more evenly.
Well, Lib, the short version is just that it won’t bake properly. It usually takes an oven about 15 minutes to preheat, which I consider time–and gas or electricity–well spent.
However, it’s worth noting that popovers are required to go into a cold oven, so you’re actually exploiting the way they warm up with the oven, to make the popover…pop. Or whatever.
I buy pizza with rising crust. It says start with a cold oven.
Duck Duck Goose, don’t some popover recipes require that the pan be red hot?
Preheating also helps seal the outside of foods faster, trapping moisture which helps food “steam” themselves inside, as well as… ta-da… staying moist. You don’t have to preheat to make meat and breads and such, but you’ll get much better results if you do.
Also, ovens that are preheated exhibit much more constant cooking characteristics than cold ovens. While my oven may take 5min to get to temperature, yours might take 10, and the cooking times will be off.
I think that the problem is also that different ovens take different amounts of time to warm up. The manufacturer of the food item therefore starts with a preheated oven when specifying how long to bake something, so that this variable is removed and consistent results can be obtained no matter what kind of oven you have.
“Biscuits.” They’re only called “bisquits” if you stop after eating just one, and from your characterization of the ones your mother baked, that probably would have been impossible. 
I find that the microwave works equally well whether or not I pre-heat.
Preheating is generally only neccesary with baked goods, or things that require precise temperature control, like custards. For things like baked potatoes or squash or chicken, or for reheating frozen foods just put them in cold.
Joe Mahma gots it rights and racer72 lives in a Bizarro World where inside is out and upside is down. 
racer72 is correct. There’s also a factor known as “oven spring” where the (breadlike) item will rise the highest when placed in a hot oven.
If you put biscuits in a low oven (which is what you’re doing if it’s not preheated; the oven takes 15 minutes to heat up and the biscuits were supposed to be done in 8) they will be insufficiently risen and tough.
However, when the temp doesn’t really matter and cooking is for a fairly long time, it doesn’t matter, as has been said. I’ve seen recipes in good cookbooks which say “Preheat oven … bake for 4-7 hours” for a meat recipe, and this is just silly.
Thanks masonite. I am considered a very good cook and baked goods are one of my specialties. And preheating is usually only required for items that use a fat such as butter, margerine or shortening as an ingredient. And if you want to bake bread with a softer crust, start the baking process at a lower temp for the first 10 minutes then increase it to the required temperature. For a hard crusty bread, start the baking process in a very hot oven and cool the the required temperature. I learned that while working at a in store bakery at a Safeway many years ago.
Muffins, if placed in a too hot oven will peak and not dome. So watch your temps.
What kind of ovens do you folks have? We have one put out by General Electric (run of the mill) and it will get to temp in less than 5 minutes at high temp. However, I appreciate the information on baking cakes, racer72.
I have a Capri gas oven, an older model, and it takes 15 minutes.
All the popover recipes I’m familiar with call for a cold oven, but I understand that there are some heresies out there. 
Racer72, don’t you think that you’ve contradicted yourself here?
I generally let the oven sit at the temperature for about 15 or 20 minutes. The reason being that you want the oven walls not just the air in the oven to get up to temperature. If you don’t then the oven temperature drops a lot when you open it to put the goodies in and takes longer to recover if the oven walls are not hot.