What's the reason to use private mode when logging on into gmail? Why not clicking onto the lock symbol in the address bar, and erasing any unwanted shit from t

What's the reason to use private mode when logging on into gmail? Why not clicking onto the lock symbol in the address bar, and erasing any unwanted shit from there, with one ore two more clicks, when done? Btw. can't reproduce. (In private mode) On some other Linux, latest stable FF, UBO (mostly default settings)