I'm pretty sure the tool is used to hold rim brakes closed (i.e. against the rim) while setting the brake pads. I found a [repair manual][1] that shows it in use on page 2. 

Should work with cantilever, single and dual pivot caliper, U and V brakes, and most other rim brakes that squeeze toward the opposite pad.  Will not work with rod or spoon brakes.

  [1]: https://bikesnotbombs.org/files/for-content/resources/training-manual/PDFs/BNBTrainingManualChapter10Braking.pdf