When cooking it is frequently necessary to know the temperature of a preparation. Sometimes it's indispensable, like for recipes using cooked sugar or foie gras.
If you intend to buy:
The classic alcohol-filled thermometer is not precise enough and cannot be used at high temperatures (to mesure temperature of something in the oven). I strongly recommend an electronic one with a wire probe. You can get a very useful model with an alarm, which sounds when a precise temperature is reached.