The other benefit of having separate controls over the sets of bulbs is that you can extend your light cycle a bit. Most people run actinic (blue) only for an hour or two when their lights come on, then all bulbs together for X number of hours, say 6 or 8, and then actinics only again for an hour or two at the end. Makes for a longer light cycle, which is nice for enjoying your tank lit up.

Like Ace and Stephen said, it's totally not necessary, but just explaining why some like to do it that way.