This rule is part of the Axis & Allies Global 1940 2E, House Rules Expansion.
The increased IPC values in the Pacific Theatre (which reflects both strategic importance as well as oil resources - see Topic that describes the Expanded Pacific Theatre) makes this Battlefield crucial for both sides - in order to turn the tide of War to their own favor.
The Increased Ship Building rule helps both sides to perform major offensives and create multiple Naval Task Forces.