Normally ignition timing at idle will not be the nominal 14 or 16 degrees, due to compensations from rpm, temperature and idle speed.
By using the 'Set Ignition Timing' command, the ignition timing will be set to exactly, enabling you to synchronize the distributor to the ECU by rotating the distributor. The timing will be set to the selected value once the Set Ignition Timing window is shown, and will return to normal once the window is closed.