The power supply converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer. In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it [...]

A DC-DC converter or another name known as buck regulator or switching regulator (because the input voltage can be either above or below the desired voltage output), provides stable regulated output voltage to supply electronic circuits.
LM2576 converter circuits perform same function as the commonly known voltage regulator LM7805 from National Semiconductor. The 7805 voltage regulator [...]

The feedback arrangement of typical switching regulators doesn’t allow the regulated outputs to go lower than the reference voltage. If you try to lower the output by modifying the feedback network, the compensation components the manufacturer recommends may no longer stabilize the regulator’s error amplifier. An external reference voltage helps overcome this problem.
Source: http://www.edn.com/archives/1994/031794/06di5.htm