From Command and Conquer Wiki

The Component Tower was a new approach to base defense that GDI utilized during the Second Tiberium War. The tower itself was just a plug into which a a defensive weapon could be installed. This allowed a GDI commander to place a number of empty towers and install the actual defenses later on. The three defense options were:

• Vulcan Cannon Upgrade: Replaced earlier guard tower defenses. Dual 20mm vulcan cannons were effective on light vehicles and infantry, but lacked range and the penetrating power needed to damage tanks.
• RPG Upgrade: Replaced the advanced guard tower's anti-tank role. Capable of holding armored vehicles at bay, it suffered from a lower projectile speed than traditional cannons, and a need for power.
• SAM Upgrade: Replaced the advanced guard tower's anti-aircraft role.

Also note worthy is the fact that component towers themselves could be deployed as part of a wall, so it opened up new defensive options.

Global Defense Initiative Second Tiberium War Structures

Buildings Construction yard · Tiberium refinery · Barracks · War factory · Upgrade center · Service depot · Power plant · Technology center Aux. structures Tiberium silo · Component tower · Electromagnetic pulse cannon · Firestorm barrier section · Seeker control · Ion cannon uplink