If you have a weak CO2 system, then the break will allow enough CO2 to build back into the system when the lights come back on.
That's the only reason I can figure that would help plants.
Algae are highly responsive to light and can do the same thing as plants with respect to light in short hour range durations.
Plants have more reserves and much greater deamnds than algae, so they can survive longer blackout peroids of several days whereas many species of algae take a beating.
This method is often suggested but it's not proven effective in relations to algae unless you already had a CO2 issue to start with near as I can tell and none of the supportors seem to be able to reconcile that issue nor get accurate measurements.
When I do it with stable CO2 levels, you can clearly see the CO2 concentration rise after the lights go off.
We often do not know what the real CO2 is in such tanks and this would explain why it works in some cases and not in others.
The thing is, if you work at reoot problem, poor CO2 supply, underpowered, too much at night near the end of the light cycle, but not
enough during the main lighting peroid, you will benefit from such a light routine.
Light drives uptake of CO2 and the downstream nutrients for the most part, but mostly CO2 uptake.
And it seems that algae are very well correlated with poor CO2 use.
I have no algae and do not use siesta's also.
I can also try siesta's and see what happens to the nutrients and the CO2 levels.
But then again, I know what to look for more than a newbie with algae issues and I try to help them make sense out of their probelsm by finding the most likely causes rather than suggesting a method based on a very dubious at best and at worst a false pretense.
You need to talk to a aquatic macrophyte speciliaitist and a phycologist, not a plant physiologist(unless they are in these specific fields). You need specific research and methods.