Giants are special units that can only be created by upgrading a city that has a level of 5 or above, or by (sometimes) examining ruins. Giants have excellent health (40) and the best stats in the game. However, they are slow, being unable to move and attack in the same turn.
Taking and holding cities. Their insane damage can 1-hit a warrior within a city wall when at full health. Also, they are very difficult to dislodge when in a city without catapults or Battleships.
Using in Battleships. Using these titans in Battleships means that your intense firepower is much harder to kill than a warrior in a Battleship.
Smashing Enemies in general. Giants deal tons of damage, and can be used as very strong melee weapons that can turn the tide of a battle.
Giants aren't good for:
Chasing Ranged units. Even though they have insane stats, they don't have dash, and that is their biggest weakness. Units such as Archers can consistently move back and snipe it without ever getting hit, and Catapults have enough range to deal serious damage to a Giant before it gets destroyed.