Giants are special units that can only be created by upgrading a city that has a level of 5 or above, or sometimes by examining ruins. Giants have a massive amount of health (40) and the best stats in the game. However, they are slow, being unable to move and attack (dash) in the same turn.
Sieging cities. Due to their high amount of health, it'll be very difficult to kill these units, especially while they're sieging. This is also mainly because of their retaliation damage, if an opposing unit were to attack a super unit, its defense would outrank the attacker, ultimately weakening that unit, if not killing it.
Battleships. All battleships have the same statistics, however, not all units have 40 health. A 40 health Battleship as opposed to a 10 health one will be much more likely to survive and continue to deliver firepower.
Clearing out tanks. The Giant has unrivaled attack and defence, and can easily outclass any other melee units. As such, they can be used successfully to clear out the defenders tanking for the squishier units and let knights through the gaps to take those out.
Ending a siege. If a unit ever successfully reaches into your city's walls, you can place down Monuments from completed tasks in empty areas of your territory, causing the city to level up, in which you can spawn a super unit, and replace the besieger with a defense mechanism.
Giants aren't good for:
Chasing Ranged units. Although a Giant has high health and damage, a ranged unit such as an Archer can easily fend off against a Giant raid due to the Giant's lack of movement.
Fighting large quantities of Swordsmen. A Giant can easily take out one or two of these guys, but will not prevail against a mob of them without adequate ranged support.
Despite not being able to be purchased, the Giant has a theoretical cost of 10 stars. This can be seen in their info card and is also reflected in the value returned when the unit is Disbanded.