The theory goes that less lights simulates the coming of fall/winter and that somehow triggers resin production. There are oft-quoted statistics on the forums saying its been proven to increase THC content in some cases.
Even if that doesn't happen with your particular strain, one thing you should definitely notice is that the color of you plants changes, to less green and more yellow. That's chlorophyll breaking down, which means you're getting a head start on curing. Worth it just for that, IMO.
Edit - whoops! 4 year old post sorry...