The role of reference voltage in buck converters?
The reference voltage is required in the feedback loop of the switch mode supply so that the switching chip has something to compare the output voltage to - it needs to know whether it has to raise or lower the output voltage.

The reference voltage is usually a high-impedance, low output current reference, so it does NOT replace the entire switch-mode supply (which by definition has a low output impedance and high current capability). A simple (but not ideal) way to produce the reference is with a reverse-biased zener diode.