From Command and Conquer Wiki
The Helipad served as a point to build, refuel and reload helicopters.
The structure was first introduced in the Great World War II by both the Allies and the Soviet Union, who required it to field and sustain AH-64 Apaches and Mi-24 Hinds respectively. It was a circular platform with armouries and fuel tanks located in integrated bunkers.
After the GWWII, the Allies developed the Airforce Command Headquarters, which could maintain multiple Harriers at once, thus rendering the helipad obsolete. The Soviets developed aircraft that could be maintained without a specific pad, eliminating the need for a helipad.