You often need to show the document to some kind of a third party, from a local government to your medical provider. People usually expect to get a nice PDF wit

You often need to show the document to some kind of a third party, from a local government to your medical provider. People usually expect to get a nice PDF with that useless skeuomorth signature and put into their archive in case they will then have to show it to somebody else during audit or something. Document+signature is simply a convenient artifact. It's obviously very easy to manipulate, so the government invented this whole PKI stuff which nobody uses, as repudiation happens almost never and nobody provides validation services for the cryptography part of it.