For similar reasons as any other field, to measure temperature. They are most often portable mercury or alcohol filled, sealed and graduated glass thermometers.
Math is often needed. For example, surgeons often need to take measurements of organs, veins, and other things in the human body. Then they could need to have to calculate the percentage of chance of...
Certain radioactive isotopes have an affinity or liking for certain body cells. Thus iodide given to a patient will bunch up and help machines "visualize" the thyroid so that a gland expert doctor...