The zerggreater spire is evolved from the spire (if there is a hive present somewhere in the same command structure) and it produces the genetic code that allows some zerg air breeds to mutate into different forms.
The greater spire enables mutalisks to mutate into guardians and devourers. It also provides the same upgrades as the spire and will retain the evolutionary progress so that any aerial units keep all upgrades that were completed at the spire.
The greater spire is available to Kerrigan and Abathur in Co-op Missions. For Kerrigan, the greater spire unlocks brood lords, and upgrades for Kerrigan's fliers upon reaching level 11. For Abathur, the greater spire unlocks guardians and devourers, as well as upgrades for the two strains.
The greater spire went through several revisions, essentially lingering in development hell. The initial designs were updates of the StarCraft I artwork which didn't seem to work. The shapes of the SC1 greater spire design didn't look good in 3D, as it was too bulky and the numerous pieces were too busy. Conveying the height the designers wanted was also a problem, since there were height limitations on tall buildings. When the development team started using Zbrush to create unit portraits some zerg buildings were also redesigned to take advantage of the more organic sculpting process. The redesigned top of the greater spire was based on an unused concept of a queen's head by Trevor Jacobs.
The revision of the greater spire made progress towards the final in-game design but the building still looked out of place compared to the rest of the zerg structures. The final unification pass finally solved many of these problems. The lower portion of the spire was reused and slightly modified with additional armor. The top portion was replaced with the new greater spire design and retextured, finally completing the long process of updating the first game's design.