I recently took a paper piecing class and this ruler was on the supply list. I don't have one but now that I've seen how it works it looks like a worthwhile "gadget" to have....and probably both sizes....6" and 12".
What I did not know about the ruler, and what doesn't show in photos, is that there is a little ridge on the bottom that lays right up against the seam and keeps the ruler in place while you're trimming off the excess. Particularly nice in paper piecing when you have multiple layers to trim as it can be a challenge for a regular ruler to stay in place.
There is also an add-an-eighth ruler which would be useful for intricate paper piecing.