Simpson Harbor was probably the best harbor in the region. For that reason, Rabaul was originally the final objective of Operation Cartwheel. However, the capture of other airfield sites and usable harbors made the capture of Rabaul itself unnecessary. Once the Japanese air and naval forces there were neutralized, and it's garrison trapped in a de facto PoW camp.
Despite the excellent harbor, Rabaul was actually kind of a crummy location for a base. The nearby volcanoes regularly dumped out ash and sulfur gasses, which significantly reduced the operating life and increased maintenance requirements for pretty much all of the equipment there, but especially aircraft.