How was this old beacon used?
It's the airfield identification beacon.


  An aerodrome beacon or rotating beacon is a beacon installed at an
  airport or aerodrome to indicate its location to aircraft pilots at
  night.
  
  An aerodrome beacon is mounted on top of a towering structure, often a
  control tower, above other buildings of the airport. It produces
  flashes not unlike that of a lighthouse.
  
  Airport and heliport beacons are designed in such a way to make them
  most effective from one to ten degrees above the horizon; however,
  they can be seen well above and below this peak spread. The beacon may
  be an omnidirectional flashing xenon strobe, or it may be an
  aerobeacon rotating at a constant speed which produces the visual
  effect of flashes at regular intervals. Flashes may be of just a
  single color, or of two alternating colors.