They can just be used for key storage. But for OpenPGP the device is normally used as a sort of an air gapped system. The secret key information is kept on the device and the operations requiring that information are done directly on the device. It is possible and normal for even the owner to have no access to the secret key information.
An advantage of a open and well documented offline cryptography standard. You can do stuff like that.
isn't a closed implementation adversarial to those benefits?
I mean any implementation can handle the signing etc, as the operations are an open standard. But running the actual pgp implementation on a system you have no access to sources, can't really see the advantage of just using an old laptop to have truly airgapped system (plugging your proprietary device on a connected computer doesn't look that airgapped to me)
