Why did US forces obtain Iwo Jima?

Answer 1

Iwo Jima is roughly halfway between the Mariana Islands and the Japanese home Islands. The US 20th Air Force was based in the Marianas after they were captured in the summer of 1944, engaged in the strategic bombing of Japan, using B-29 heavy bombers.

Iwo Jima is just big enough for a decent sized airfield. With Iwo Jima in Japanese hands, Japanese fighter planes could fly from it to attack the American bombers going to and coming from Japan. And the Japanese on Iwo could radio warnings to Japan that an raid was on the way, so the defenses could be prepared, anti-aircraft cannon manned, defending fighters in the air.

Once Iwo was captured by the US these early warnings were no longer given to the Home Islands, and US fighter planes, which had a shorter range than the bombers, could be based on Iwo to escort and defend the bombers on the way in and out of Japan.

[Further note: it turns out that fighter escorts were of very limited usefulness, as the almost total annihilation of the Japanese air defenses meant escorting fighters were unnecessary. Also, the early warning station on Iwo Jima was supplemented by others which were never attacked, so the loss of Iwo Jima did not impact the Japanese's early warning system at all.]

But the main value of Iwo turned out to be as an emergency landing strip for bombers damaged over Japan, or experiencing mechanical troubles. It was nothing but vast open ocean between the Marianas and Japan, and any plane in trouble was going to go down in this vastness, and the crew would be very lucky to get picked up. About as many B-29 crewman were saved by using Iwo as an emergency landing strip as there were Marines killed taking the rock.

[Note: the last paragraph is highly disputable - postwar analysis showed that the vast majority of "emergency" landings on Iwo Jima were not for real emergencies, and instead for training or minor issues. In addition, search-and-rescue operations in the region could easily have been handled from other small islands in the area, which were unoccupied by Japanese. So, the number of U.S. bomber crew lives saved by the Iwo Jima airfields is likely very, very few. Probably the best justification is for emergency landings for the atomic bomb missions, since a loss of either bomb due to plane malfunction would have been a huge blow, as replacing the a-bombs was a very time-consuming process. ]

Answer 2

It was to serve as an air base for repair, refueling, and offensive sorties against Japan. It was probably not worth the cost (the Pacific has a great many islands).