Are there use cases where a signature itself needs to be signed?
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.