The answer is the almost universal... "it depends".
I've noticed that almost every recipe starts with "pre-heat the oven to xxx". Is it necessary? I'll agree with others, and in some cases, I'd say yes.
Some foods need to go in at the stated temperature to turn out properly, usually a matter of getting a good crust. Sitting for 10-15-20 minutes while the oven goes from room to 350-450 or whatever could really throw off the results. Especially if the total cook time at the set temperature is a short time.
In other cases, the recipe writer wants to try to give a specific cook time, and ovens vary so much in preheat time, it is safer to just say 'preheat, and bake for xx minutes. You already have the variable that different ovens don't all hit the set temperature, and there are hot/cold spots, why add another variable?
For other foods, I'm sure it doesn't matter (something that's going to be in for several hours), but writers are lazy and just say 'preheat' out of habit.