The primary reason is because the United States has failed to convert to the metric system. So, one example would be if a new vaccine was manufactured in Canada and distributed in the U.S. On the packaging, it could say to store it between 2 - 8 degrees Celsius. Someone who doesn't know how to convert to Fahrenheit might just think "that sounds like what the weatherman said the temperature is in Minnesota", so they put it in the freezer. A pharmacy technician needs to quickly convert it in their head and put it in a refrigerator. Thereby not destroying the vaccine.
Other examples basically follow the same logic, and it's usually about storing drugs, monitoring patients body temperature, and compounding substances into solutions.
Perhaps, someday the USA will convert to metric. It sounds confusing to most Americans. But, it's actually much more streamlined than the hodge podge system we use now.