Timing an oven to start while I am out - but what about preheating? Many ovens have a timer which allows you to set the start time, so you pop your item in and later in the day it will automatically start cooking it unmanned. 

But all the oven cooking I can think of requires I preheat the oven before putting anything in it.

Is it that preheating is desirable rather than essential, or is it that the timer feature is designed for those few cases where preheating isn't required?
Preheating is mainly needed for breads and anything that has a short cooking time (&lt; 15 mins or so). I don't bother preheating when I'm roasting something for a longer time. If you're setting a timer, I assume this is something that will take a while to cook, so I wouldn't worry about preheating.