In most cases, YES! The semiconductor junctions in an LED (which produce the light) require very specific electrical power in order to operate properly. If the voltage (electrical force) supplied to the LED is lower than required, very little current (electricity) flows through the junction, resulting in low light output and poor performance. If the voltage is too great, too much current flows and the LED can overheat and be severely damaged or fail completely (thermal runaway). Configuring an appropriate LED driver for each application is essential for maintaining the optimum performance and reliability of the LED(s).
Also, different types and models of LEDs have different voltage requirements even though they may share the same current rating. Beyond that, the voltage required to achieve the proper current can vary with temperature, composition of chemicals in the LED, etc. Therefore, LEDs with the same part number may have variations in the precise level of voltage required for proper operation. Generally, it is desirable to regulate the current flowing through each LED to the rated current in order to maximize the illumination and life of each costly little LED. To do this, some type of true power regulation is required. A resistor can be used as a cheap solution to restrict the current flow, but since a resistor turns power into heat to do its job, the resistor often wastes more power than the LED saves while adding to the heat that needs to be removed.
There are a few applications in portable lighting where a driver is not completely necessary and a simple resistor is used (however, this is not recommended!). This is the case for many multi-cell flashlights using LEDs, since it is a very cheap solution. By placing the correct resistor in line with the LED, the available rate of power from the batteries is restricted to a level low enough to be tolerated by the LED. Since the available power from the battery will decrease as it is consumed by the LED, the light level will begin to drop as soon as the unit is turned on and power is taken from the batteries. The major disadvantages to using a resitor on an LED are linear light drop-off (light level begins decreasing as soon as power is switched on), poor efficiency (the resistor is consuming much of the power), and no thermal protection (the LED may overheat and there is nothing there to stop it!).