Yes, it uses less power. From Popular Mechanics:

"Dimmer switches save energy by switching the light on and off 120 times per second, which reduces the energy moving through the circuit. The longer the switch is off, the lower the light output and the greater the energy savings. To accomplish this, the switch uses a semiconductor device called a triac (triode alternating-current switch)."
Source(s):
http://www.popularmechanics.com/how_to_central/home_clinic/2812616.html

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