To follow up, Taiwan literally blocks their ability to see and defend against attacks coming from behind the Taiwanese island. It’s about observability and reducing easy beachheads.
On a more macro issue, it's also existential for the CCP because it shows the Chinese people (I'm talking the entire worldwide diaspora of people who call themselves Chinese) that there are indeed ways forward socially, culturally, technologically, economically, and most importantly, politically that are valuable to society that don't involve the CCP.
Hong Kong was a threat because it showed Chinese people directly that economic growth was possible without the CCP.
China has the biggest Navy in the world and they regularly circumnavigate Taiwan. They're not just sitting there on the mainland with binoculars wishing they could see what's on the other side.
