There might be situations when you want client-side crypto not for sending data over the network, but to securely encrypt a document that you want to send / use in a different context. Using an emscripten-compiled crypto library would allow you to do this in your browser without sending any of the data over the wire.

Alright, but everything can still be tampered. At best, you can cause a denial of service, at worst modify a document that a user signs has its own.