The first reason is definitely food safety. Even if you are a seasoned cook you cannot always tell if a food is properly cooked inside just by looking at the colour on the outside. Food may appear to be cooked but may not be at a safe temperature. The internal temperature is the best indicator. Harmful bacteria can be found in all types of food and it is only killed at certain temperatures. Taking proper precautions in the purchasing, storing and preparation of our foods are only some of the steps in ensuring our food remains safe. The required temperatures will vary depending on the type of food.
The second reason is so your food is cooked to its desired optimal state, not overcooked, still tender, juicy and flavourful. Using a thermometer allows you to determine when the food should be removed from the heat source, while still taking into account that the temperature may continue to rise as it rests and personal preferences.
The third reason is that it is so easy to use. Using a thermometer allows you to take the guess work out of cooking many different types of food, dishes you do not regularly cook all the time, bigger items such as large birds and roasts, and using different types of ovens where temperatures may vary.
There are many different styles of thermometers so instructions should be followed for each one. Placement of the thermometer is also very important to get an accurate reading and is usually in the thickest part of any food without touching any fat or bone.