The battery (or power supply) generates power, the LED and resistor both use power, but they do so in different ways. The LED uses the power to make light (more power, more light). The resistor does not make light, it makes heat (more power, more heat). And as you know from the last quiz, any voltage left over from the LED is used by the resistor. That voltage & current in the resistor is lost forever as heat and doesn't do anything useful in our circuit. Since it's inefficient to just pump all our battery power into the air as heat, we should make the power used by the resistor as small as possible, and the best way to do that is to keep the voltage low.
The upshot? If you need to make an LED brighter, adding batteries is wasteful: you're better off using a smaller resistor! If you are making up a power supply, by adding up AA's in a pack, try to have about half or one volt minimum 'headroom' above the highest forward voltage, so that you can have a small resistor, around 100 or 200 ohms. Going lower than that isn't suggested because the forward voltage can vary, and resistors can vary, and the battery can vary and all these little variances of 0.2 Volts or so add up and you won't get the brightness you want.