Actinic bulbs reproduce the 420nm blue light found at deeper areas of the ocean, but need to be used in conjunction with 6500k or 10000k daylight bulbs. Once you get up to 14K and 20K bulbs the need for actinic is reduced or eliminated depending on the bulb as they produce enough of the 420nm spectrum already.
Actinic bulbs also help the colors on your corals and fish to fluoresce.