With this option you can move your off-hand slot next to your hotbar to the right so that your armor hud can stay right next to your hotbar regardless if you have something in your off-hand or not. And vice versa for left-handed users (left hand can be enabled through Options > Skin Customization > Main Hand).

This suggestion is for people who want their armor hud to always be towards the left, or for left-handed users to the right. Adding this option helps make more customization possible, and that can make it more likely for people to download the mod, thus resulting in more downloads for the mod, obviously.