This is from the Simpsons. (but a different episode than Bev's answer above) Danny DeVito is the voice of the character that says it.
"What does it do?"
"It doesn't DO anything. That's the beauty of it!"
It's from The Simpsons in the episode where Homer's half brother (I think, maybe step brother) Herb Powell is explaining his invention. He is talking about the 'drinking' bird and Homer asks, "What does it do?"
Can't you hear Danny DeVito saying this?
The movies for me that came to mind when i first heard the quote were, Flight of the navigator, One of the Ghostbusters movie, A Gremlins movie, and the simpsons. I haven't checked them out yet, but what i get from the quote is that it is from a comedy that is still kinda serious, not like airplane or caddy shack or a comedy like that.
by the way, Bernatrick, it isn't from Van Helsing because first he asks what it's for and the monk says I don't know and VH says you don't know? You've made this thing and you don't even know what it does? And the monk says well of course I know what it does. You only asked me what it was for.
or something along those lines. 😗