If the 20th Air Force Urban Redevelopment units are flying out of the Marianas, they need it because of the reason IOTL. You have it almost at the right spot for emergencies and for escorts to fly from.
In TTL Tinian and Saipan have been captured by the Americans in December 1943. However as fester posted recently the B-29s aren't ready. If fact earlier in this storyline fester had posted that the B-29 program would follow an ATL schedule similar to OTL. This is due to the PODs in Keynes Cruisers not greatly influencing the development of the B-29. So you would not expect the B-29s to begin operations until about May 1944. @fester If I've misremembered this detail about the B-29 in your TL please correct me.
So from Tinian to Tokyo (a useful reference point) it's about 1500 miles. And Iwo Jima lies very close to the flight path and also close to the halfway point for missions flown from the Marianas. Iwo Jima would be an important objective in those circumstances.
But despite the Northern Marianas being in American hands 7 months earlier then OTL they aren't useful for the bombing campaign yet. Six months further along in May 1944 in TTL the B-29s maybe beginning operations. They could operate from Saipan and Tinian. But what other things might have changed by then? I would think by that point Clark Field would be available for B-29 operations. It's not any closer then the Marianas for missions targeting Japan though. From Clark AFB to Tokyo is about 1800 miles.
But Okinawa is ideally situated for B-29 operations. From Naha to Tokyo is about 950 miles. This is close enough to permit fighter escorts for the length of the entire mission.
All of Japan can be reached from Okinawa by B-29. Also Korea and most of China. It's a large island with plenty of room for the construction of large airfields in the coastal areas close to the ports and harbours.
It would take a full scale amphibious operation to take Okinawa. But in TTL with the growing strength of the USN and the heavily depleted IJN, much more depleted then OTL at this point, I would think the capture of Okinawa is a reasonable and practicable objective in TTL for an ideal base from which to conduct the strategic bombing of Japan. Could it be taken in the first half of 1944?