Because the only known Japanese space center is located here, Saga prefecture is a very important part of not just the Southern region, but of the whole post-Great Battle Japan. It is possible that there are other space centers, but those are yet to be shown.
Saga prefecture Space CenterEdit
Saga prefecture Space Center serves as a launching and landing site for SSTOs that allow sending goods and information across the ocean, most particularly, SSTO maintains a 'trading route' with United States. Since the space center is located on the shore, in order to protect it from the possible attacks from the "Fleet of Fog", a small fleet of human ships is stationed there. However, even a single light cruiser, Nagara, was able to obliterate that fleet and would have destroyed an SSTO had not I-401 sunk it.