Every time I read a API that uses signed/authenticated requests (AWS, Let's Encrypt ACME) I wonder exactly the same thing - why is this needed in the first plac

Every time I read a API that uses signed/authenticated requests (AWS, Let's Encrypt ACME) I wonder exactly the same thing - why is this needed in the first place? If TLS guarantees lack of replays it seems to me like signed requests just protect their own complex infrastructure from reusing the same request twice...