Ovens vary. So it may come out undercooked, or overcooked, even if the recipe says ‘preheat and then bake 30 minutes at 375º’ and you do exactly that.
One of the things novice cooks do need to learn is how to check whether the food is done; and not to trust the recipe time precisely, but to take it out sooner if it’s starting to burn and stick it back in if it’s raw in the middle. Or (and where) to stick a thermometer in it shortly before you expect it to be done.
I’m going to say, baking is one thing that you really have to preheat the oven for. When you put your batter in the tin into the oven, the temperature then is quite crucial for the final result.
Maybe you get away with it if you’re baking biscuits, but cakes don’t like it.
Roasting meat can also require it. You want to get a good outer crispness, then you need to put it in at quite high temperature, say 210 for 15 minutes, then drop back to 170-180 for the rest of the cooking.
I’m surprised not everyone knows that.
I roast meat at 350º or sometimes 325º all the time, and start it in a cold oven. If the meat’s got a skin layer, that’ll get crisp just fine. If it doesn’t have a skin layer, I don’t particularly want it to get crisp; though I understand that you do. Tastes vary.
I’m surprised so many otherwise smart people mix up singulars and plurals (“criterion” vs. “criteria,” “phenomenon” vs. “phenomena,” “syllabus” vs. “syllabi,” even “woman” vs. “women.” And things like “parent’s” vs. "parents’ ").