What differences between redirect withInput() or just back() in Laravel

I'm wondering how withInput() works in laravel, because if I look inside the code it populate data to flash session, then I tried just redirect()->back() and i could retrieve the old input with old() helper, so why I need include withInput() if I didn't filter some inputs must be flashed or not eg. redirect()->withInput($request->only('email'))

via Chebli Mohamed