The Starport is an advanced building constructed by Terran. Its primarily serves to construct air units.
Even if most of the other air units are not built, the Starport is required to build the Science Facility which unlocks the Science Vessel and is required to research Tier 2 and 3 upgrades as well building a Covert Ops and Physics Lab which unlock the Ghost and the Battlecruiser respectively.
Similar to the Factory with the Machine Shop add-on, when multiple Starports are built, the Terran player can elect to build Control Towers on a subset of the Starports. This can occur during heavy Wraith production strategies, where only one Control Tower is necessary to research the Wraith Cloak and Cloak Energy upgrades, leaving the standalone Starports sufficient for construction. Because the Starport sits toward the end of the Terran technology tree, it is usually reserved for the mid to late game.