Well, this is more of a military defense strategy question, but it does have roots in the Monroe Doctrine. The Monroe doctrine basically states that any military action from the Eastern Hemisphere (Europe and Asia) into the Western Hemisphere (The Americas), would be considered an affront to the United States. This makes sense from a defensive standpoint: the U.S. likes to keep oceans between them and any invading armies (see: War of 1812).

So, in order to keep with that doctrine, you need to be able to defend the oceans from any invading force. While the US's massive coastline is nice, from a defensive strategy standpoint it is always better to have an outpost away from your main base. This allows quicker deployment, better supply lines, and it makes your enemy attack the outpost first (otherwise they will be attacked from the front and the back, which is a large disadvantage).

So, Hawaii is one of the more perfect outposts in the pacific: it is a set of large islands that can support a population. It can grow its own food (so it doesn't need to be constantly shipped supplies), has large coastlines with high points for surveillance, and is well situated near the middle of the ocean.

Furthermore, it also increases what are called "spheres of influence". This is a term used to describe how countries exert power over populations. With Hawaii, the US gains a sphere of influence all around the pacific islands.

Well in the olden days of coal-powered ships you needed coaling depots were your ships could take on supplies of coal to continue their journey. Hawaii was therefore strategically important as a coaling location (as were most of the other Pacific Ocean territories taken by the US). Since it is also the largest piece of sovereign US territory in the Pacific it made sense to have it be the primary base for the Pacific Fleet. The Army, Air Force, and Marine presence followed the Navy.

Historically also Hawaii was the starting point for the Pacific Campaign (along with Australia) and a lot of infrastructure was built up to support the war effort in the Pacific (similarly in Southern California, Alaska, and Fort Lewis-McChord). After the war it was the western defense line against the Soviets along with being a key supply station/first stop on the way to Korea/Vietnam when those wars were active).

In the modern era it’s because its (relatively) close to Asia and is sovereign territory which means we don’t need to ask a foreign power for basing rights (like in Okinawa & Korea). So if a major war was to break out in, say, Korea the US would first send troops based in Korea, then the units stationed in Okinawa, with the first major reinforcement coming from the forces stationed on Hawaii (of which the Army Division is set for rapid deployment by air and sea).

For your question about our interests in the Pacific, well our #1 trading partner is China, which is on the Pacific coast. In fact the current planned strategic military plan for the US Armed Forces is an increased focus on the Southeast Asia region with increased emphasis on the bases in the Pacific (Hawaii, Korea, Okinawa, Alaska, the various islands like Guam and Samoa).