Aircraft carriers are effective in taking out ground troops as well as buildings. If these carriers are used in large numbers the can pave way for yor ground attacks.
Aircraft carriers spawn many fighter planes which can swarm any building and take it out. Targeting the enemy production sites or their military buildings will cripple their fighting ability.
Carriers can also help support your ground troops. Airplanes from the airbase have limited range but carriers are mobile so they have many times larger, if not world-encompassing, range.
Powerful as they are, Aircraft Carriers are still vulnerable to Submarine and Heavy Ship attacks, so carriers should be protected with Light Ships and Submarines of your own.