Manchuria must have seemed to be an excellent addition from the Japanese point of view. A good degree of key ressources which the island lacks, good internal shipping routes ,a relative short 'Chinese' frontier (considering the extent of the region), considerable strategic depth towards the Soviet Union and a relative small population ( in Chinese relations). Keep in mind that for a long time it was 'the' heavy industrial heartland of China with a hefty percentage of the GDP. This is before the big boom transformed the Chinese economy completely.
It is difficult to understand why they extended their forces so much into China. I guess it was a case of too good a chance to pass, of complete victory just a step away, of not giving in to problems.
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