Generally speaking, ovens will put out as much heat from the heating element as possible to reach the desired temperature quickly. Once at that temp, the oven maintains a largely even and consistent temperature so long as you aren't opening it repeatedly. That allows you to have the same temperature surrounding your food at all times and have predictable cooking results and timing.

If you put food in at the start without preheating, your food is surrounded by room temp air instead of heated air, yet is exposed to high temp direct radiant heat from the heating element. It will eventually reach the even temp expected, but only after several minutes of that initial exposure to the direct heat of the element. That is far more likely to lead to over cooking the surface exposed to the heating element and/or undercook the remaining surfaces and interior. It is closer to trying to bake in a grill than in an oven.

If the thing you're cooking is thin and is fine to heat from largely one side (like a pizza), your results may be acceptable if you keep an eye on it. If it is a cake or something though, it will be hot garbage, either burnt or soupy. Regardless, the cooking times on the instructions will not be remotely accurate.