Why is it that on some directions it says something like "preheat an oven for 400 for 10 minutes"?  Is it not enough to preheat the oven until the oven signals (i.e. usually with a beep) that the desired temperature has been reached? Are there any advantages to waiting longer even though the oven remains at the same temperature? 
Many ovens, particularly older ovens, do not beep.  Indeed it is not uncommon that they have no temperature indication at all, particularly on gas ovens.  So the preheat timing for an oven is usually an indication from the author of the recipe that the oven needs preheating with a guide to how long you might wish to wait, usually intended as a minimum.  I have known some recipes to instruct food be put in the oven before temperature is reached for various odd reasons, that's not common, but does happen. 