How and Why the last 8 bytes MUST be overwritten in TLS 1.3 as described below if negotiating TLS 1.2 or TLS 1.1?

In RFC 8446. About the random in ServerHello send by Server.

In 4.1.3. Server Hello

32 bytes generated by a secure random number generator. See Appendix C for additional information. The last 8 bytes MUST be overwritten as described below if negotiating TLS 1.2 or TLS 1.1, but the remaining bytes MUST be random. This structure is generated by the server and MUST be generated independently of the ClientHello.random.

Why and How ?

The last 8 bytes MUST be overwritten as described below if negotiating TLS 1.2 or TLS 1.1