Responses should also be signed in a similar way to the requests so that client's can verify they are talking to the correct server.
Compromise of a server itself would mean the secret is compromised and therefore all communication is compromised.
But it is possible that a domain is hijacked instead.
In this case, the server can simply accept all messages it receives, then send malicious messages back which a client will blindly accept and act upon.
Responses should also be signed in a similar way to the requests so that client's can verify they are talking to the correct server.
Compromise of a server itself would mean the secret is compromised and therefore all communication is compromised.
But it is possible that a domain is hijacked instead.
In this case, the server can simply accept all messages it receives, then send malicious messages back which a client will blindly accept and act upon.