I read on another post just yesterday that it was also for receiving aeration and that vendors turn off the pumps until they PE for photos. Dunno how true that is, but when you think about vendors always having pics with PE without a cloud of food for PE it makes some sense.Polyp extension happens for two reasons that I'm aware of. One, the polyps extend because they're depositing the minerals/elements they're pulling from the water below themselves, which is physically how a calcifying coral grows. Two, they're trying to filter feed.
Considering that the minerals/elements used in growth for corals plateaus and starts to rise as soon as the lights are off, I'm going to guess they're trying to filter feed now that they're not getting food via photosynthesis.