OpenPGP is generally used for encrypting and signing e-mails.
Are there any other use cases?
If yes, which cases?
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OpenPGP is used for file encryption and signing as well. Code signing is - by extension - also possible.
PGP is also sometimes used for (automated) application level protocol, for instance by ProtonMail. Although that is a mail protocol, it is still somewhat different from explicitly signature generation by the user.
OpenPGP has a rather modular message and key format. Reading through the spec may reveal some other use cases. The commercial implementation of encryption products by Symantic could be interesting as well.
Generally I do see more generic use of CMS (Cryptographic Message Syntax, also known as PKCS#7) within a Public Key Infrastructure (X.509 certificate based, as used in TLS/HTTPS, in which case it is known as PKIX).
CMS is used for XML signatures, government identification/authentication, mail, PDF's etc. It's even more extendable/flexible than OpenPGP which makes it possible to use it for more purposes.
Note that flexibility is kind of orthogonal to security. For instance, for signed SOAP it is possible to sign one part and process another because there are multiple ways to refer to the data.