Everything a server needs to choose parameters is in it: supported_versions, cipher_suites, supported_groups (10), key_share (51), signature_algorithms (13), server_name (0), ALPN (16), pre_shared_key (41, always last) and more. Because it is unencrypted, it reveals the destination host name (unless Encrypted Client Hello is used) and a fingerprint of the client software.
Size matters again: a hybrid X25519MLKEM768 key share adds about 1.2 KB, so a modern ClientHello often spans two TCP segments or QUIC packets. Some servers, load balancers and firewalls that assumed it fits in one packet failed when browsers enabled post-quantum key exchange, a class of server bugs documented at tldr.fail after Chrome enabled hybrid Kyber key shares by default in version 124 (2024).
GREASE values (RFC 8701) and Chrome's random extension order (since early 2023) keep servers from ossifying on exact contents. The TLS fingerprint tool shows what your browser sends.