Are cooking thermometers essential?
Thermometers are really practical when you do not have a lot of experience cooking things; it let you keep track of the proper food temperature.

For example, I do not cook beef roasts often, so I will use a thermometer to not mess up my cooking and waste a good amount of money on a good cut of meat.

Thermometers are essential for confectioneries where exact temperature are needed, mostly for sugar caramel stages.

Thermometers are also essential if you decide to try "molecular" cooking techniques with different chemicals; where temperature needs to be precise for certain reaction to happen (I do not have examples for that).