A lot of people look for linear power supplies that will run projects from 1.8 volts to 24 volts with higher currents. It is easy to use off the shelf linear regulators, but when the voltage range is so high the power dissipation in the regulator leads to disappointing results as all the power between the lower voltages and the bulk power supply are dissipated in the regulator. So even with a huge heatsink these regulators can't supply much current at the low voltages. For example a 3 amp regulator running at 5 volts may not supply more than 1/2 amp due to thermal cutoff.
This supply uses a low cost switcher to keep the input to output voltage across the regulator to a comfortable 5 volts allowing for more current and smaller heatsinks.
The current for the design here is limited by the transformers to 1 and 2 amps depending on the regulator used (LM317 or LM350).
It provides for low noise of the linear with some of the power savings of the switcher.