I'll also add the the federal US government makes extensive use of personal TLS certificates, and they store the private key material on a smartcard embedded inside your ID.
Your cert is used from everything from signing and decrypting emails to gaining access to sites to signing attestations that something happened.
Partially true. TLS stands for "Transport Layer Security". TLS certificates are used on servers to encrypt communication. The certificates on the smartcard are used for digital signatures, encryption, and authentication. All of those certificates follow the X.509 certificate standard.
Your cert is used from everything from signing and decrypting emails to gaining access to sites to signing attestations that something happened.