1. Chip production in Taiwan that is essential to several industries. You would want to at least destroy that production capability if China invaded.
2. Taiwan allows China the capability to push further out into the China/Phillipine/etc. Sea increasing their strike capabilities against neighbors.
3. Our image took a beating after Afghanistan. We still need to project power in that region if we would like to contain China. I see a new cold war developing with them. It would be extremely costly for us to attack each other, so they will be looking to exert power in other ways. A committed U.S. presence (read: we are willing to literally bleed) in the region helps convince Japan, South Korea, Australia, the Phillipines, Vietnam, India, etc. that we are a reliable bulwark against Chinese aggression in the region. If we are not, then it maybe becomes more in their interest to start siding with the Chinese at our expense.