/opt/retropie/configs/all/retroarch.cfg →video_rotation = 1 (90°) or 3 (270°)This is why you need a vertical pack. These games are unplayable on a horizontal monitor without massive black bars.
When setting up a vertical ROM pack, you will encounter two main aspect ratios: vertical arcade rom pack
If you are playing on a modern TV or monitor without physically rotating it, ensure your emulator settings are set to "Integer Scaling" or "Pixel Perfect" to avoid stretching the image, which ruins the art. This is why you need a vertical pack