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.