eWell, incase u accidently add or remove a mod, you can press the undo. It could also be used for mistakes that were made during a launcher setting that the person made, which they can undo.
The redo is basically the opposite. If you undo an action then you realise maybe it's the wrong one, you can redo it. Just like uninstalling something, undoing then redo it.
In conclusion, i feel that this would be very important to the CurseForge community.