Answer:
1. Money! Hawaii was rich with tropical farmland. If the US got its hands on that land it would grow the economy and produces yummy fruits.
2. Military Reasons. At the time the Pacific West Coast of the US was protected by military ports up and down the coast. If the US got Hawaii it would help protect the pacific west coast of the US. Allowing ships to stop for rest and fuel on their way to Asia, allowing advanced warning of an incoming attack, etc.