This escarpment is an ancient fault-line. When the scarp first formed, the land that now holds Perth dropped below the level of the rest of the continent. As time moved on, the seas covered the plains of Perth, which is why Perth is such a sandy city, having spent many years on the bottom of a sea-shelf.
These days, the Darling escarpment is well-known for it's high quantity of bauxite (or Aluminium ore), which is generally mined by the company Alcoa and shipped to the bauxite refineries to the south of Perth.