Spawning Goblins and Spear Goblins together would cost 4 Elixir, which is 1 Elixir more expensive than deploying them as one card. Using the Goblin Gang not only saves on Elixir, but also saves a card slot - the player can use another card.
However, using both Goblins and Spear Goblins as two cards in the same deck has the advantage of being able to be deployed separately, such that they cannot be countered all at once.
However, the Goblin Gang can be used as a The Log-bait or Arrows-bait card, considering it's not wise to ignore the Goblin Gang, as they deal heavy damage to Crown Towers.
They can also be split behind your King's Tower to attack both sides.
Zapping the Goblin Gang will kill the Spear Goblins, leaving three Goblins to take out.
Because of their placement, the Goblins in front, which have more health, will tank for the Spear Goblins behind, creating a mini-push, though the Goblins actually deal much more damage than the Spear Goblins.
An Electro Wizard or Valkyrie can make short work of this card and will keep plenty of health for a counter push.
This card can provide a decent distraction for enemy high damage single-targeting troops like P.E.K.K.A. or Prince, giving time for you to finish these troops off. However, they are vulnerable to splash damage, and the enemy would likely support the tank with splash troops.
The Goblin Gang can be used to surround and destroy non-360 degree splash units as they won't be able to kill all the spawned Goblins at once.