Taiwan is an unsinkable aircraft carrier. It provides a massive platform from which to attack the mainland. Maintaining control denies this advantage to any attackers.
FWIIW it was historically used by forces which threatened Chinese sovereignty, including Japan. Since Chinese leadership actually reads history instead of making stuff up as they go along, as Westerners do, that's probably why it's such a sore spot for them.
Another thing to keep in mind is that Taiwan consists of about 70% of mountainous areas, i.e the perfect terrain for an organized resistance.
Invading Taiwan is but an easy task and it is believed to be an endeavor that would surpass the D-day.
[0] https://books.google.com.tw/books?id=mzwGDAAAQBAJ&pg=PT117&l... [1] https://foreignpolicy.com/2018/09/25/taiwan-can-win-a-war-wi...