MovieBuff09

Chosen answer: Because Shandor believed that human society was too sick to survive (to quote Egon). Gozer was supposed to destroy human civilization in the hope (apparently) that something better would rise to replace it.

Grumpy Scot

Chosen answer: They cross the streams to cause a reversal in the particle flow in Gozer's dimensional gate, shutting it down and causing a massive explosion. Gozer, having taken the form of the Stay Puft mascot, subsequently burns up in the explosion as the marshmallow he now is.

Garlonuss ★