The PLANs amphibious warfare ships will be used to lift the Chinese marines to the Pescadores Islands to seize that position as a staging area for helicopters and air cushion vehicles to shuttle follow-up forces to Taiwan itself. This will also have the effect of nullifying the anti-ship missiles based there.
Today, Strategypage emphasizes this point as well:
Discussion of China’s plans to invade Taiwan often ignore the smaller, Taiwan controlled islands, that the Chinese invasion forces will either bypass, or hit first, on their way to Taiwan. The smaller islands, Quemoy and Matsu, are within artillery range of the mainland. But a group of larger islands, the Pescadores, are within artillery range of Taiwan itself. The Pescadores have an area of only 127 square kilometers, and a population of 90,000. There is also a military garrison on the island, including an armored brigade, plus anti-aircraft missiles and mobile anti-ship missile units. The Pescadores are doubly important, as they are opposite the most important landing beaches on Taiwan.
Of course, quickly getting troops on Taiwan itself in large numbers means the Chinese can't afford to wait until the Pescadores are captured, as Strategypage quite correctly notes. Speed is China's answer to our naval and air superiority. If China wins fast then our intervention is moot.