An LED is a rectifier (Light Emitting Diode), it drops a voltage when you put a current through it. They are rated to give a specific light output per current passing through. Normally, when you light an LED, you have a specific voltage you are driving it from, say 5V or 12V. You have to put a 'dropping' resistor in line with it to limit the current and drop the rest of the voltage. Resistance_value = Source_voltage - LED_voltage / LED_current. This is very inefficient, as most of your power is lost across the resistor. The switching power supply delivers a set current, whatever current your LED is spec'd at, and the voltage will be determined by the LED itself, so there is no other power lost in the circuit, except the regulator circuit, which is in the neighborhood of 95% efficient, as opposed to an Resistor Limited circuit, which would be about 40% (5V source), or 16% (12V into 1 LED) or 33% (12V into 2 LEDs in series).