There where multiple different motives for the US to annex the Hawaii Islands, including economic, militaristic, and strategic motives.
Hawaii was a place that offered excellent conditions for production of high quality cash crops, especially sugar cane. The sugar cane was bringing in a lot of profit, so the US wanted to incorporate these islands because of the economic interest first.
The islands are located midway between North America and Asia, thus they have a good strategic location. What was going to be produced in Hawaii could have just been sold in Asia, thus lowering the expenses for transportation. Also, the islands were very useful as a place where a port can be built, and where the ships will be able to recharge.
The militaristic aspect was also important. Hawaii is located further west from the United States, so it provided nice conditions for building on a military basis which act as a first defense and warning line, or to be used for recharging and attacking.
