If a giant is killed before it reaches the opponent's tower, a quantity of zombies will appear in the location of the giant's death, as well as a handful of slimes and cola cubes. This is done to add additional value to the mob given its enormous cost, since there's no feeling as sinking as watching your precious giant die for nothing. It also adds a level of strategy to slaying giants. For example, it may be beneficial to your team to allow an enemy giant to begin destroying your castle before killing it, as this will prevent mobs from spawning in its place. Similarly, it adds to the importance of dealing with giants as soon as possible, so that you have more time to kill its minions before they reach your castle :)