Measuring temperature in a kitchen is important to ensure that food is cooked as desired and is handled and served in a safe manner. Using visual indicators to judge temperature is an inherantly unreliable method which can result in food being under or over ccoked with dangerous or unpleasant results.
Thermometers are a critical food safety tool in commercial kitchens with multiple purposes including ensuring food is cooked to safe temperatures , food is stored at cool enough temperatures, and that dishes and utensils are washed at high enough tempertures.
In some instances such as tempering chocolate where temperature needs to be accurate within a degree or two a thermometer is a very valuable tool which makes life easier and helps ensure success.
There are many different types of thermometers which are used in cooking and the general types are described here.
Candy / Fat Thermometer
Fridge or Freezer Thermometer
Digital Probe thermometer
Disposable temperature indicators
Pop up temperature indicators