I’ve seen it used on Cinema Camera filters to make funky reflections in the lens.
We were filming a dream sequence and to make the edges of the image soft and blurry, we used an optical flat (a clear filters basically a piece of clear glass that slides in front of the camera’s lens) and the DP (director of Photography, aka the Cinematographer) smeared some Vaseline over the edges of the flat, painting the blurry edges with his finger. It worked really nicely, unfortunately I can’t find the final video online to show the result.