In the client-server architecture used by the SSH protocol, a client can authenticate by providing a tunneled clear text password or by using a public/private key pair: this is called public key authentication. A user who logs in via a public key on a remote machine has complete access to the command line; in certain situations, however, it may be useful to associate a public key to a single specific command, for security reasons.
Having a reliable backup of our GPG (Gnu Privacy Guard) secret key is not optional: the key represents our identity, and loosing it could potentially be a disaster. Creating a backup of our keys and sub-keys is quite a simple thing to do using gpg, and the resulting files can be easily backed up on one or more devices. Electronic devices such USB drives or hard disks, however, tend to fail, and usually in the most inappropriate times; therefore as an extreme resort, we may want to print our keys to paper.