Sometimes you have a photo which you took, say, in 16 by 9 proportion, and you want to print it on, say, 4 by 3 paper, without cropping. Or maybe you want to use it as a wallpaper on a screen of a different proportion. For that, there's letterboxing: adding a border on two sides to adjust the proportions. This app does that.
Photo Letterboxer is basically a universal filter. It shows up in your sharing menu as a share target; you can share any JPEG or PNG to Photo Letterboxer, pick the proportions you want, and the color of the border, and press the Letterbox button. You'll be given another share menu; choose an app, and that app will start up and be given the letterboxed image. (If you want to save it to a file, I recommend my Save To File app.)