Answer

By default OSD settings, monitor will detect video signal source automatically so users do not have to open OSD menu to select input source.
However, if "Auto Scan" is disabled in OSD menu, user can open OSD menu to select input source when monitor does not receive video signal. It is because disabling "Auto Scan" will let monitor stay at last connected video source, if monitor is connected via a different video cable, user can open OSD menu to select the corresponding input source.