I use them basically for what indy said above- so that when my lights turn off, it doesn't get pitch-black and freak out the fish (they used to start darting around like mad). Plus I have 'em cause they look pretty cool at night when I have company over. I'll have my normal lights go off at 9:00, then actinics until 10:00, then moonlights until 11:00. It's just my preference, though.
125 gal. reef w sump/refugium & 8-bulb 640w T5's
45 gal fresh w/plants + CPF lighting