If you are cooking a large whole animal, you have to keep track of BOTH the internal meat temperature and the temperature at which the meat is cooking (ambient temperature). So you stick a meat thermometer into the meat and check it from time to time. Or you use an instant-read thermometer and check it from time to time.
If you are using an oven or enclosed cooker or smoke, you will probably have a thermometer that will give you the pit cooking temperature (although these are usually not very very accurate and are taking readings from higher in the cooker and not at the level of the meat). Otherwise, how do you determine the pit temperature? Stick and oven thermometer on the cooking grate (a better solution, actually)? And if you are using a rotisserie, then what? Strap and oven thermometer onto the meat?
Our solution is to use one thermometer that can be inserted into the meat for constant monitoring of internal temperature and that also can read the ambient temperature through a sensor on the back of the dial. Both readings are visible on the front of the dial so at a glance, you can see both at the same time!
Essential little tool, very useful, for not much money. We do not start a big rotisserie cook without at least two of these handy.