Secure Messaging and HTTP Signatures

Sorry if this is a noob question but I've been looking at Secure Messaging
and HTTP Signatures specs

Am I right to say that for signatures, the signature is in the HTTP
headers, but for secure messaging the data is stored in the message body.

If that's correct, why are these done in different places?  The reason that
I ask is that in crypto currencies such at bitcoin the signature appears in
the message body, rather than, the header.