I don't understand add rotation or scale (Cntrl+A, last step of the video), I tried with 2 cubes one rotated and one no, I...

I don't understand add rotation or scale (Cntrl+A, last step of the video), I tried with 2 cubes one rotated and one no, I thought it was to apply objects others rotations or locations but It does not work. thank you very much.