I am always looking for ways to save money. Even if it's just a few cents here and there, it can really add up. For instance, let's say your oven uses $0.15 cents per hour (running at 350 degrees). You have your oven on for an hour - baking time - and a half hour - preheating. That will cost you $0.225 a day to bake. (That's if you use your oven once a day at 350.) Which equals out to $6.975 a month. Not too bad.
What if you didn't preheat your oven. You would save $2.325 a month, or $27.90 a year. Now this is pocket change to some, but saving $28 for just not preheating your oven? I'll take it. :)