The Apollo Starport and Immigration Facility, as it is officially recognized, is otherwise known my a myriad of names and titles. Some include but are not limited too, the starport, military base, or even just mil base. The area serves key roles in all three campaigns.
In Easy Company, the area is the sight of one of the first missions, Investigate the Apollo Starport, and comes under attack by a large horde of zombies. It also is area through which the Security Cameras side mission is activated. The area will also contain supplies from deceased Alpha Company soldiers, which make for useful early game supplies for easy company. Lastly, it houses one of the Power Generators used in the Restart Generators mission.
The area is the initial landing zone for Alpha, and their dropship is found nearby. The hold the area from initial waves of enemies, and regroup with Lt. Sheng along with some of his Security Team Remnants. The area is later on of the areas Dr. Hidetoshi Tanaka will flee to in an attempt to escape, making it a common location for the battle with Cerberus.
The area isn't used for much of the Apollo campaign. During the initial chaos of the map, several Marines will meet their end at the hands of Devourers here. HAL will also hack the Sentry Guns if they were not destroyed. The area will also make one of the potential hiding places for Dr. Bergmann after he escapes. Lastly, the area houses the massive Ivax walker. While initially allied, it is quickly hijacked by HAL who sends it off on a murderous rampage.