If you are going to roast a whole animal (pig, hog, lamb, goat) you will need to keep an eye on temperatures for various parts of the roast. Since different sections are much thicker than others, they will have to be treated differently.
If you are using a rotisserie, you will probably make the fire hotter at both ends and less hot in the middle to compensate for this. To be able to calibrate your heat source better, its nice to have a constant reading from both the fore and hind quarters (you get 2 Dual Sensor Meat and Oven Thermometer) and to occasionally check in the ribs and loin. This combination of 3 thermometers will do all of that.