I'm thinking about building a Ghost Sniffer as my first prop. But, I have 1 question. Does anyone know what it was for? I think the only time it was ever used was in the scene with Peter & Dana in her apartment. I don't think its true purpose was ever revealed. All Venkman ever said that it was technical, it's one of our little toys.

And that's basicly these props are, toys.

It actually one appeared in the RGB comic! It was designed to sniff out and detect spooks.

Here is the RGB 1989 write-up or the Ghost Sniffer, however the name is different.

Ghost Detector:
Also known as a “Sniffer”, this samples air from a possibly haunted locale and analyses it for any free-floating ectoplasm.

Hope that helps.