I used one until recently in my darkroom. I believe that it can be used to gauge what contrast setting to use as a starting point in addition to the more simple use of exposure setting, which is how I used it.
When an exposure is to your liking, make a measurement of a particular part of the image and record this and the exposure time for future ref. A print of any size (within reason, I guess) on similar paper can be accurately set up by adjusting to give the same light reading.
Once some experience is gained, it is easy to use measurements from similar images to give a good starting point for other prints.
I now use a similar device, the Paterson EM1030, which is a little more flexible for my work.
