[dheera]

LEDs are usually current-controlled, not voltage controlled.

You might have several LEDs in series and they might very well have a voltage drop across all of them of 24V or even much higher when maintaining an operational amount of current.

[petschge]

I know. But at the maximum rated current the voltage drop across the LED is going to be only marginally higher than the band gap. Definitely not 24 Volts. So the question becomes what do you do with the excess voltage. If you are smart you don't just burn it in a resistor. And at that point the question is exactly what the OP asked: For which input voltage do you minimize total losses.

[bluGill]

The could, but in general you would prefer to wire them in parallel so that nobody notices when one burns out - it just gets a little dimmer.