It's an anti-rotation tab, which is designed to fit into a small hole on a panel or scratchplate so that a) the pot still works (for a while at least), if the nut comes loose and b) the pot body resits rotation so the fixing nut is less likely to come loose in the first place. Some guitar manufacturers do make a hole in the scratchplate to locate this tab, but it's far more normal to just get a pair of pliers and snap it off so it sits flush.