The logistical bases set up in the Philippines were absolutely vital to the two Operation Downfall invasions. And the decision to make those Operation Downfall logistical bases goes back to the 1944 Philippines base (FILBAS) agreement.
Page 304 of WASHINGTON COMMAND POST: THE OPERATION DIVISION lays out the various Pacific command conferences of
WW2. There were two Washington DC theater commander -- JCS conferences in March 1943 and Feb-Mar 1944 and four Joint Theater
Commander conferences in the Pacific:
Pearl Harbor - Jan 1944
Hollandia - Nov 1944
Guam - Feb 1945
Manila - July 1945.
When you cross reference WASHINGTON COMMAND POST with Coakley and Leighton's GLOBAL LOGISTICS AND STRATEGY 1943-1945 to find out what the subjects of these conferences were about, since all high level military strategy is logistical in nature. Chapter XXIII showed the Nov 1944 conference at Hollandia was primarily about the coming Luzon campaign and it hammered out the "FILBAS" Agreement (See page 566).
The agreement had the following points:
o MacArthur would support the XXIV Corps staging to Okinawa from
o The Navy abandoned an earlier promise to crew US Army small
coastal freighters, the Coast Guard would instead
o MacArthur would create the logistical infrastructure to stage nine
POA Infantry/Marine divisions for future operations against the China
o Nimitz gave up any further claim to US Army service units in the
closed out South Pacific theater
Of those points, only the first two were carried out in full. Mac could never get his shipping squared away in time for Navy needs for the larger Formosa operation that Iwo Jima and Okinawa later replaced. So Nimitz started requesting the Army South Pacific service units again.
However, when you look at the logistical staging for Operation Olympic, you realize a couple of things. First, MacArthur never stopped trying to fulfill the FILBAS logistical objectives. There were exactly nine infantry divisions staging from the Philippines for Operation Olympic.
Second, two of those Olympic divisions were being staged from the Southern Philippines.
This fact wasn't just a matter of "that is just where they were they were" after MacArthur went haring off after liberating Filipinos.
There were hard logistical limits on both the port capacity of Manila after Japanese destruction of facilities there in Feb-Mar 1945 and what could go over the beach and small ports of Leyte.
In addition, the limits of even world class port clearance facilities of the time made ships of more than 6,000 tons and more than four cargo holds "problematic" in terms of rapidly unloading cargo via cranes and cargo nets. It was not until the ISO container revolution that you got the expansion of cargo ships to the Midway class carrier sizes we see today.
The two divisions to be staged from the Southern Philippines for Olympic had to be staged from there because that is where the port capacity was, and all the islands in between needed to captured to allow unimpeded sea movement for MacArthur's small freighters, impressed Aussie fishing smacks and tug-barge shipments to those ports prior to that staging.
The Southern Philippines campaign never made it to the JCS level for reappraisal after Luzon. It happened because Nimitz's sign off for it in Nov 1944 FILBAS agreement.