pkexec uses the default authentication agent on every distro. So if you call a GUI-app on a desktop it will ask you for your password in a nice interface that looks like all the rest of your desktop environment. If you call it on a headless machine it will ask for you password on the terminal. sudo can just ask at the terminal. Back in the days when BiT had two GUI for Gnome and KDE we used gksudo and kdesudo to do the job. After removing the Gnome GUI and migrate the KDE GUI into Qt I needed a common way to launch the GUI on all desktop environments. pkexec does exactly this.The permission escalation with PolKit is something different. It allows to run sub-routines with higher permissions but keep the main app at user level.