Are there use cases where a signature itself needs to be signed? I'm writing some code to digitally sign certain files (JSON, if you must know), and I'm trying to understand the use cases, not being a crypto guy myself.

I understand there are use cases where several parties all need to sign the same content, e.g. the two parties who enter a contract sign it to indicate their approval.

But are there use cases where B signs not just the content (of the contract, or whatever), but the concatenation of content plus the previously-created signature by A of the content?

Update: given this discussion, I created a proposal that I believe covers the case in case you are interested: http://upon2020.com/blog/2014/03/digital-signatures-on-json-payloads-lets-call-it-jsonsig/
In a nutshell there are two main uses cases for signing an existing signature:


validation: the signature of another person (ex: a superior) is
required to give effect to a primary signature. The second signature
covers the content, the first signature and potentially additional
data added by the second signer. 
witness / notary: a second person signs only the first signature, to
vouch that the person applying the first signature is indeed the right one.
Contrary to the previous case, the content is not covered by the second signature, because the second signer does not commit to it.