Without an instant read thermometer, you can waste a lot of oven heat with the door open as it slowly ticks up to finally settle on the correct temperature after about a minute or so. But with an instant read, like this Dr. Meter Meat Thermometer, the thermometer only spend 4-5 seconds calculating, letting you get back to cooking without too much energy waste.
This model is not your favorite, but it is way cheaper than the Thermapen at just $9 today with code J5GIWWA9 today.