If you're referring to why Japan chose to occupy a large area as opposed to just the "Southern Resource Area," it was all about their defense strategy.

Japan's defensive strategy was predicated upon the idea that the US would mass a large naval force to retake the Philippines. This large force would almost certainly be several times larger than anything Japan could field. To make up for this, Japan planned to use the many Pacific islands as airbases (i.e. unsinkable aircraft carriers) to make up for the quantitative difference. The idea is that this mega US fleet would be attritioned down by air raids and submarines, and that by the time they arrived, the rested, supposedly qualitatively "superior" Japanese fleet would then sortie in a sort of "mega-Jutland" and destroy the US fleet in a massive naval struggle, sometimes referred to as the Mahanian "Decisive Battle," named after the naval thinker Alfred Mahan. This victory would then cause the US to sue for peace.

Thus, Japan's plan of conquest was to acquire enough such territory that they would be in a position to sufficiently attrit the Americans. The more time they could buy, the more resources they could extract from the Dutch East Indies, and the more planes they could deploy in anticipation for this battle. The problem was they didn't know exactly where the battle would take place. To offset that, they created a defensive perimeter centered around the major Japanese bases of Truk and Saipan, so they would be just as able to respond to an American invasion from the Southeast (the Solomon Islands) as well as the North (Aleutian and Kurile Islands). This perimeter contained many islands without significant natural resources but at the same time could be used as airfields to base planes off of in preparation for the decisive battle.