Spawning Goblins and Spear Goblins together would cost 3 1/3 Elixir, which is 1/3 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.
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.
It can be risky to deploy the Goblin Gang on one lane, since The Log can take the Goblins and Spear Goblins with ease, causing an Elixir advantage. However, the Goblin Gang can be used as a Log-bait or Arrows-bait card, considering it's not wise to ignore the Goblin Gang, since they can deal heavy damage to Crown Towers.
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.
The Goblin Gang can be used to surround and destroy non-360 degree splash units as they will not be able to kill all of the spawned Goblins and Spear Goblins at once.