yes, for many items you can just put it in and let it heat up and add extra time. casaroles, baked beans or frozen meals, for example, you can do this.
But, for baked goods, the oven needs to be at the baking temperature before you put the food in. SO, you turn the oven on while you mix up the cake. It is the first thing you do. You set the temperature to the baking temperature, for example, 350'F. How do you know when to put the food in ? Well, usually the oven has a light or some indicator that tells you it has reached the temp., my toaster oven beeps once when it has reached the temp. In my big oven, since the thermostat is a bit off, I have a dial thermometer in there so I can check.