The Aircraft Carrier has become an essential part of the Navy. Its purpose is to bring the battle to the enemy's doorstep. It has a range of 15 miles with its artillery type guns. The range of its fighters is over 100 miles! It has many AAA guns (Antiaircraft Artillery) since World War II and the Japanese Kamikazee units that sank many ships in the war.
The use of the destroyers lately is massively decreasing. They are mostly used as convoys for carriers. The massive guns of the destroyers could launch a 2 ton shell up to 30 miles. They will still be used for submarine hunting and other things such as protection and expedition boats. To clear water lain minefields.
Ships need many people to operate especially carriers which are like cities. The Navy has a high budget to keep these ships and new ones and pay for new recruits.