Holly has it right. It was used to look at stereoscopic pictures, about postcard size, that were fitted into those two metal holders. When you looked through the eyepiece, the two images blended into one 3-dimensional one.
I guess this makes me an official "antique," as I actually had one as a child. My eye doctor prescribed it as a fun way to do eye exercises. The pictures, usually of mountain scenery, etc., had little numbers on them, and you had to start with number 1 and follow the path to the highest number, usually about 20. I liked it, and considered it sort of a toy. Whether it actually improved my vision is highly doubtful.