"Try explaining QNH to the man in the street"
See that hill over there - the map here says that it is 2500ft above sea level. When the altimeter reads 2500 or less with the sea level pressure set we will hit that hill.
See that airfield over there - the map here says that it is 625ft above sea level. When the altimeter reads 625ft we will hit that runway.
Most common cause of initial IR rating test failure is pilots failing to reset QNH in the missed approach having made an approach using QFE - UK CAA.
It's an altimeter not a heightameter!
Regards,
DFC