Komac
Your "pure speculation" is just that pure speculation...and incorrect.
The extra frequency allows transmissions to be removed from the primary Aerodrome Control frequency, and allows the Aerodrome Controller to concentrate on controlling traffic in the control zone without having the distraction of issuing IFR & VFR clearances to aircraft on the ground or airborne outside of the control zone.
The current procedures didn't happen overnight...and are the product of extensive customer consultation, trial, and amendment. Those of us that use them day-in-day-out find that they work pretty well, and allow us to move a large amount of traffic...more than any other tower in the country that deals with (primarily) IFR/VFR training traffic and IFR RPT operations. I believe we are the 2nd busiest tower overall and we do that between the hours of 6am and 8pm, wereas the busier tower (and the other 2 INTL towers) are 24/7!!