The theory is that by flushing all available nutrients from your medium, you force your plant to use stored nutrients in order to , "go into emergency mode" in order to finish reproduction (being an annual).
Their is an opposing camp that bielievs cannabis can't store nutrients in this way, and they say that flushing is depriving the plant of essential nutrients at an essential time.
Unfortunatly most of this is based on, theory, conjecture, anectdotal reports, and commen wisdom instead of research (to the best of my knowledge, maybe I don't know about some research on this). Their is though, the fact that many of the most famous growers in the world recommend flushing, which is worth something IMO.
Also, personally, it kind of makes sense to me from an evolutionary biology point of view. It seems to me that an annual plant producing buds late in the season would be evolutionarily programmed to respond favorlby to the common conditions of that time of year, which in most cases involves much more rain in the fall then summer. But that's just my own untested theory on why cannabis would like to be flushed at the end. You can decide for yourself.