Same thing they gained from invading Hong Kong, they think it belongs to them.
Or as one of my old friends told me while playing Final Fantasy 12; the only legitimate reason to wage war against another country - land.
Hong Kong is already part of the Chinese mainland and was already kinda part of China, but Taiwan is a geographically strategic location that puts both Koreas, the Philippines, & Japan in a tougher position. Even without war it would make trade and travel in the Pacific much harder.
I loved the mystery of the original run of Pokemon. The campaign to catalog all the animals that fell into the new Pokemon classification has so much missing information and unexplainable details.
Meanwhile in the kitchen
LIFT THE LID BEFORE YOU PEE