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 from the middle to the end, 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 on the battlefield.
It should not be rushed early on in the game as by itself(or 1-2 more) it can easily be beaten by a group of heavy soldiers-this being said, a capped out group of tanks is most likely going to beat a capped out group of heavy soldiers. However, if you are in a 3v3/4v4 (or in a confident 2v2v2) match, having one player rush light tanks early on and phasing into heavy tanks later is a viable strategy.
It is usually a good idea to place Heavy Tanks in front of soldiers due to their high health and ability to soak up a lot of damage. However, this can cause problems while moving medium to long distances as tanks are slower than infantry.
Like 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.