Fireball is one of the strongest spells in the game. It damages any unit within its area. Similar to the Arrows and Rocket, it has a travel time speed. The closest you are in the Crown Tower, the faster the projectile will be. Fireball only costs 4 elixir with really strong damage that can kill low health to medium health troops.
The benefit of having a high level Fireball early is a great advantage. It can easily destroy lower level card troops like the Witch, Wizard, and Three Musketeers. You can also counter the benefit of the opponent’s Elixir Collector by using Fireball, hitting it with the opponent’s nearest Arena Tower.