It is a complete package. The basic goal is to capture enemy bases in order to secure your own position or undermine your opponent's position. Bases allows you to project naval and air power further and further in the Pacific. Bases provide forward LCU staging areas for further invasions aimed at yet another base. Airfields are important because the primary element of control in the game is the air group.
Air groups exert a zone of control around the bases at which they are stationed.
If your bases are protected by friendly air zones of control, they will remain supplied and continue to prove a thorn in your opponents side. Conversely, if you can envelop an enemy base in your air zone of control, you can cut it off from easy supply, starving any LCU's stationed there and preventing air replacements. A base with few working air groups and weak, unsupplied LCU's is a prime target for amphibious invasion. To play Pacific War, you have to view the whole Pacific strategically. Your ultimate job is to decide which base to capture next.