Tzar is a super heavy tank. Equipped with a powerful AoE plasma cannon can obliterate whole groups of light units with one shot. It is very heavily armored and can take in massive amounts of damage.
It defines a great use in an early T2 game.
Take it to the front covered by some AA units in order to push through the remaining T1 units and defenses like a knife through butter. Though, better try not to lose it, because it leaves very precious wreckage (894 metal).