A thermometer is needed when brewing beer on a couple of occasions. Our beer recipes kits require you to steep malted grain at certain temperature ranges. A thermometer will allow you to hit target temperatures precisely and therefore brew a better tasting beer.
You also need to know the temperature of the beer before adding any yeast. The yeast you add also respond best in certain temperature ranges. If you pitch the yeast when the beer is too warm the cells will die. Knowing the beer temperature before pitching the yeast is simple and quick.