I've just got back from a painting course, painting landscapes on site. I had trouble judging the first tones and my distant mountains were too dark. One of the other students got out one of these little beauties and showed me how to look through the small hole. Instantly I could see that in fact my mountain was too dark! I had been judging the colour against the bright sky, but when isolated next to the mid grey of the view catcher, I could judge the correct colour and tone of the mountain really well. I bought myself one of these and then was shocked to see how cheap it was on Amazon. I should have looked here first. Never mind. At least you are looking in the right place. My version has a small hole in one corner of the rectangular frame as well as the hole in the sliding plate shown here. This makes the use of the tone-finder hole even easier. Mine also has markers for popular board sizes for outdoor work: 12 x 16, 8 x 10, 11 x 14, 8 x 12 and 9 x 12, so if you use these formats it is much easier to use this than, say, a pair of cardboard Ls or a camera as you will have the exact proportions of the edges. These formats are for inch measurements but since they are ratios they work with cm too.
Handy modification: I didn't think I would use the actual viewfinder section as I use my camera as a viewfinder BUT, if you stick a piece of clear plastic film to one side of the outer frame, you can mark it with a grid of thirds or quarters, for example, to help you draw more accurately. Use a permanent marker that works on plastic and make sure you leave it to dry thoroughly.