Most rear lights for bikes mount on the seat post, but under the seat is also where the popular "seat wedge" bag that holds basic repair tools goes. The seat wedge can obscure the rear light.Also, the Knog light that I bought and like points largely at the ground when mounted there. It has a wide enough throw that it's not a huge problem, but I figured I could solve two problems at once.I designed this thing to solve these problems. I can now mount both my rear light and my wedge bag and have full visibility of the light, and the light is straight up and down.