I actually cook a lot and I do this. The benefits aren’t very significant but it certainly doesn’t ruin meals (if you cook like me lol).

It’s more energy efficient and doesn’t affect the food as much as you would think. Because of the temperature difference between your oven and house, and that heat rises, tons of heat flies out of the oven as soon as you open it. That is why there’s the massive blast of heat from the oven when you open it, it’s practically flying out at mach speed.

Afterwards there are only minor benefits from preheating the oven because the temperature has dropped significantly and it has preheated itself (and food) again anyways.

Secondly I never strictly use timers to tell me whether my food is cooked or not. Even “30 mins at 300 degrees” on different ovens will give you varied results, because built in oven thermometers are not very accurate. Additionally to make the oven look more accurate the screen will lie to you. On my oven, quickly after reaching temp, it may say “400”. If I turn my oven off and on, and immediately set it to 400 again it’ll say something like “325”, because it’s a lying sack of shit that tries to trick me into thinking it’s faster and more accurate than it really is.

That’s why independent oven thermometers are so popular. This is also why I gauge my food by it being cooked or not. I look at the crispyness and browning of the food. If it’s a good color, I remove it. If undercooked, no shame in returning it to the oven. If the outside is burnt and the inside undercooked then that’s the fault of either the recipe, cooking vessel or temperature (which is part of the recipe anyways).

Thirdly it makes managing things marginally easier. Instead of trying to optimally time your preheat as to not waste time, but also not to waste energy for 20 minutes, it’s easier just to turn it on with the food in.

I hope you understand why I don’t believe in the satanic oven preheating conspiracy.

Because in a recipe it’s impossible to specify cooking times without pre-heating. It’s easy to say 10 minutes at 200 degrees, because this would be exactly the same for everyone. Every oven is different, so the time would be different depending on your oven, which the person writing the recipe can’t know. So if the instructions on something like bread say pre-heat and bake for 10 minutes at 200 degrees, they know the result would be good.

There is also the fact ovens warm up differently. If there is a heating element within the compartment where the food is being heated (especially above), this element gets way too hot and emits a lot of infrared radiation whilst heating up the oven. It does this because it wants to get to the set temperature as fast as possible. Once it gets there it only needs to maintain that temperature, which is much easier, so the heating element gets much less hot during this time. If you set something like a cake in the oven with a heating element right above it, best case the top of the cake gets baked more than the rest, worst case the top gets burnt before the inside cooks.

Then there’s the fact whilst heating the temperatures inside the oven fluctuate a lot, some parts get hot fast, other parts take more time. When you have food that’s sensitive to that you def need to preheat.

And there’s a lot of chemistry going on, for example some foods get really greasy if they don’t get hot enough while cooking. Whilst these food could be cooked with the temperature going from 50 - 150 degrees, the end result would be much better if it’s just cooked at 200 during the whole process.

Now there are a lot of cases where this doesn’t matter and if you know your oven well enough you can compensate. But there are plenty of legit reasons to pre-heat and you may even have better results when pre-heating, even if the end result was fine before.

So I agree, people should pre-heat and there are tangible benefits!