Well, maybe.
It's an industrial-strength way to vary the voltage going into an LED circuit you might build. It allows you an easy way to check the performance of such a circuit over the entire range of possible battery voltages. It generally will not help you in determining how much current is actually being supplied to the LED, unless you have an extremely simple circuit (a single resistor, for example).
Of course, 3 or 4 sets of batteries at various stages of discharge are one hell of a lot cheaper.
/frank