From watching the tute it seems the purpose is to machine stitch those folds of the mitered corners rather than sew down by hand. Perhaps for quilts to be judged this might be useful, but not on my must have list, right now anyway.

I agree that it looks like they have made some simple hard

For me, I think I could live without but looks interesting.

I had been thinking about doing this with corners just recently, but never actually tried it. It makes sense that there would be less bulk in the corners and they would always be accurate and perfect.
The way I do corners now, sometimes they come out perfect, a lot of the time almost perfect, and a few times, bad. I think I would like to try this.