Guam figures heavily into announced Chinese naval strategy. They have laid out by decade when they plan to push their seapower out to the "inner" and "outer" island chains. Guam is the key to the outer chain. It has the biggest all weather port between Pearl and Manila, weapons depots for the Navy and AF, B-52 runways, sub bases, etc etc etc.
It is directly under the Chinese cross-hairs. It is the key to any American defense of the Western Pacific.