I didn’t know much about this battle, but it makes sense, from a Japanese perspective, to try and control these islands.
Japan was in trouble once the USA took Saipan and US bombers could hit Tokyo. Attu doesn’t appear to be that much farther away.
The Japanese began building an airbase on Shemya (I've been there) about 30 mile to the east. The Americans finished the base, and it was packed with P-38's and B-24's. There were 20,000 GIs there. They bombed Honshu around the clock. The suicide rate among the GIs was quite high. There's girl behind every tree. There are no trees.