Nothing really, apart from the fact that most people who purchase bitcoin, do it on some centralized exchange and almost all of them require KYC. So unlike the bad title OP chose, the centralized service has KYC not bitcoin.

The importance of it comes down to how much the user believes in financial sovereignty. For most people, it is all about making some profit in which case KYC is not as big deal (which is why CEX is still popular!) but for anybody who seeks financial sovereignty, any form of surveillance (that includes KYC) is in complete contradiction.