It depends on the plant, commercially they grow some <"salad crops under 24 hour light"> ("continuous illumination"), but others need a "night" and they use a 16 hour day for them.
My guess is with the coming of LEDs, there is now a lot of work on R:B ratio and on LED pulsing. It may be that a series of light periods, with small dark gaps, increase yield and decrease energy use. One of the issues with discharge light (HPS) is that they don't re-strike etc. meaning that you have to have them constantly on during the programmed photo-period. A very flexible light could be linked to photo-sensor and come on and off as PAR levels vary from clouds etc.
It only needs to be a marginal gain to be commercially viable.