In article .com,
wrote:

In todays times, as someone has mentioned, the only use I can see for
the markers is as follows. For situations when the Kollsman wndow is
'blanked out' one could take the local given pressure, convert that
into an equivalent altitude, dial it in with the reference markers,
then read the correct altitude above MSL from the normal hands.

The use was to set field pressure Altitude on the reference markers so
that your altimeter reads QFE... that is altitude above Field Elevation.

Simplifies flying the pattern at 800ft, 1500, or what ever is normal for
your airport, but the real use is for approach where your minimums are
200 ft... instead of looking for a DH of 5,500, you would be looking for
200.

American and Eastern used them this way for decades. In older aircraft,
one altimeter was set to QNH and the other to QFE for approach. Later, a
third altimeter was installed and both pilots would look at QFE on
approach.

When you only fly to one or two airports in a day, it's not that big a
deal, but when you are making 6 to 8 landings with low ceilings,
descending to the 200, 300, 400, or whatever is on the plate simplifies
things a lot.

Over the long haul, it's nicer to find concrete than foliage.