What is the meaning of the crosshatch marking on altimeters?
It looks like the crosshatch (or barber pole) indicator doesn't have a consistent meaning (a similar marking is used on some airspeed indicators, by the way). I found several different examples of when it can appear:


Below 10,000' (FAA PHAK; Army RC-12H manual)
Below 15,000' (instrument service catalogue)
Below 16,000' (another instrument catalogue)
Above 0', i.e. above sea level (instrument repair information)


The only source that gave any explanation is the last one:


  When the altitude is below sea level the barber pole is no longer
  visible. This is provided to avoid the error of reading –1,000 Ft. as
  being +10,000 Ft.


After following a bunch of links and skimming some discussion forums, my conclusion is that @RalphJ is probably right: manufacturers added the flag to highlight that the aircraft is at a relatively low altitude, in a way that's much easier and quicker for the pilot than 'parsing' multiple hands on the altimeter face. And the fact that different sources give different altitudes highlights an important point: it's always a good idea to read the handbook or manual for all your instruments because you can't assume that everything works the same way in every aircraft, even something as 'simple' as an altimeter.