Remember that the USA already used an “island hopping” strategy of bypassing and starving out every Japanese garrison that it could. So the question is why Iwo Jima got listed as one of the islands that the USA did need to take as an air or naval base. ETA: more or less, the closer US forces got to Japan, the more important every island became and the less scope there was for bypassing. Before August 1945 everyone presumed that the war would never be over until the Japanese home islands were invaded and conquered, and all strategy was geared towards making that possible. I’ve never heard that Iwo was valuable as a logistics base but presumably it had some capacity in that role.