When you turn on a burner on the stovetop, you have a pretty clear idea of what’s happening. You can see the gas flame or feel the heat emanating from an electric or induction coil. Ovens are different. You have to take the appliance’s word that it’s really at 350 degrees and not secretly sneaking up to 400 degrees and about to scorch a batch of cookies.
That’s why you need an oven thermometer.