The mysterious Ivo Shandor finally reveals himself in sci-fi movie legend Sigourney Weaver) inhabits in New York City, and on top of which the final battle between the Ghostbusters and Gozer takes place.
Shandor's architectural work was seemingly always in service of facilitating Gozer's rise, as he served as leader of a cult dedicated to the otherworldly being. While Shandor was thought to be dead during the original movie, Ghostbusters: Afterlife revealed that he had kept himself in some kind of stasis inside the mining complex (i.e., Gozer's temple) he had erected in Summerville, Oklahoma. His awakening is seemingly triggered by Gozer's ascension, as he can be seen briefly moving around before the quartet of proton streams Egon Spengler (Harold Ramis) set up in the temple pushes Gozer temporarily back down.
When Gozer finally does fully ascend, their dutiful worshiper Shandor rushes to them, hoping to rule the world together. Instead, Spider-Man trilogy.
Simmons briefly returned to the Jameson role in Up in the Air, and more. With that in mind, it's unsurprising that Reitman would reach out to Simmons to play such a brief, but memorable role - one that's so centrally important to Ghostbusters' mythos.
Interestingly, if past Ghostbusters lore is any indication, it's possible this might not be Simmons' only opportunity to play Shandor. In the 2009 Ghostbusters video game, which featured the original cast back to reprise their roles - including Harold Ramis prior to his death in 2014 - and was promoted as a sort of unofficial Ghostbusters 3, Shandor was one of the main antagonists, despite being a ghost himself. While Ghostbusters: Afterlife doesn't seem to be keeping the game as canon, there's no reason they couldn't still use the Shandor as a ghost plot device in a potential Ghostbusters 4, should it happen.
