it assists with the bio load on the tank, gives it a day to catch up. most fish in the wild will go a day or two with out eating, I am unsure if this rule applies to bettas as well.
I tried the fasting on my community tank a couple of weeks ago to clear up the water a bit but I had to take my betta to the side and feed him because he was throwing such a fit in there
I know , I know , a fish throwing a fit? but trust me he was thrashing around and if it didn't have a lid on it I'm certain he would have jumped out with all the racket he was making, I heard him hit the lid a few times and big splashes and he looked sincerely p.o~ed
BUT I have heard that bettas should only eat a few times a week, never tried it myself and mine OBVIOUSLY are accustomed to daily feedings
so is mine he will throw a fit if he dont get something once a day I catch mine fruit flys all the time I buy bananas just so I can get a few for him everyday he stays mad in the winter months because they are no fruit flys around I have to give him blood worms in the winter just to make him happy I believe if they are a way to spoil a fish I have managed to do it I cant have not one pet without spoiling it to something my kids are grown and I dont have anything better to do with my time except spoil the fish and the cats
My boys have 2 days a week that they get nothing.they seem fine. Both disease free . I was told by a betta breeder that they can actually go a week without anything without any ill affects..now that I would never do IMO that is too long without nurishment in a captivity. But the weekly fasting does help the bio load.