An island with little immediate value to Japan could still serve as an American logistics hub. Leaving it unoccupied allowed Seabees to arrive with merchant shipping and a small escort and begin establishing an air base without a battle. A Japanese garrison removed that easy option and imposed the costs of an opposed landing. Once an attacker had to fight for an island anyway, choosing a larger and more useful objective became more attractive. The value of occupation therefore included the time and resources it forced an opponent to spend.
Notable Point · 2:22:46-2:24:44
Why Japan occupied islands with little immediate value
Occupying a small island could deny an opponent an easy airfield and force it to spend time and resources on an opposed landing.