A heavy tank is an original unit from the game, The Conquerors. It is well known as a great unit for its reliability for Raids at such a reasonable price, the downside being its relatively slow speed.
Mostly between the beginning and mid-term games, people will buy heavy tanks to prepare to fight a rival team. People like to use heavy tanks together with anti-air tanks due to it being a good combination to stop soldier armies and planes due to planes being commonly used in the battlefield.
Due to tanks being slow but good in attacks, it is good to try and make them go in the back of a soldier army to do more damage due to tanks costing more money than other units.
Mixed tanks with heavy solider will form a great attack strategy
Alike all "Heavy" units, the main hull of the tank is covered in metal plates, possibly to show it has greater power than its predecessor.
Even if it appears with more armor, the damage is the difference, not the health.
Heavy Tanks are the most commonly used tank unit in TC3 and for a good reason.