Using your phone in one hand and a pencil in the other, you save yourself the trouble of buying a traditional bubble level and even measuring tools. The grid makes it easy to match the relative spacing of the images and center the images in the available space. The angle finder feature makes it easy to measure the angle and make sure something is aligned.
If the lines look a little uneven (not quite straight) when you first open them, just press the calibrate button and follow the instructions. Translated instructions: simply place the phone on a flat surface with the screen facing down after pressing the button until the timer expires.