Single action harps can be a first - low budget - step in the world of the pedal harp. Historically the single action harp ( invented in 1720 ) has been the precursor of the double action concert harp ( invented 1812 by Erard).
In most countries it has been replaced by the double action harp that offers more tonal possibilities. However it survived in Austria and the south of Germany, beeing widely used in the folk music of these regions.