He said this was the saddest song ever. I think he was right
The deus ex machina of the Ghostbusters crossing the streams of the proton packs helped them to—spoiler alert—defeat the Marshmallow Man and the evil demon Gozer at the end of the film. According to Ramis, this activity didn’t appear in script. He and Aykroyd were unsure how to get the Ghostbusters out of the final scene alive, and because the nuclear technology behind the proton packs was “explained” with humorous techno-babble and mostly left up to the audience’s imagination, they came up with the idea of crossing the streams—an act which would somehow cause a cataclysmic shift in our dimension. After this decision was made, they added in some foreshadowing of the event to an earlier scene, only to revisit the concept in the climactic standoff at the end.