The reason for doing so is to reduce the current flow between the two silver electrodes. You can get some pretty high current flow with 3 batteries hooked up in series. The higher the current that flows between the electrodes the larger will be the solid silver colloids & the less useful they are for bio-availability.
The most useful colloids are the smallest ones you make which is why you should stop the process long before the water begins to turn to a white cloudy color. As a rule, I do not take internally CS I make that has any color in it, it is all but useless internally to the body & will simply be excreted, but it is useful for topical applications or to put in your shampoo or body wash.