Suppose you have a 12V supply and need to power a 5V device. If that device is a simple resistive load like say an incandescent light bulb, you can simply use the right size resistor to drop the voltage. If the device is more complex and does not draw a constant current, the resistor obviously won't work. Something like a 7805 would be the easiest solution. In both cases you are wasting a lot of power. If the first case over half the power is just generating heat in the resistor. I assume the same, or close to the same, amount of power is lost in the 7805 regulator.
If you needed to light two 5V light bulbs then you could hook them in series and waste a lot less power by using a smaller resistor.
Now for my question, can you do something similar by using 2 7805 regulators hooked up in series to power 2 separate loads? Alternatively and more what I want, can you also have the outputs hooked up in parallel to power one 5V device?