With a dimmer, the light intensity can be tailored to the needs of plants at each stage of the lighting cycle. During growth, plants need less light, so a dimmer allows electricity to be saved.
Without a dimmer (i.e. at full power), the Spectrabox UFO has a 40-50% larger ‘footprint’ during the growth period than during the flowering period.

LED Dimmer 0-100%, for UFO Spectrabox