Many tasks require “chaining” ssh sessions. Consider my example from earlier: I ssh from my workstation to the dev server. While there, I need to perform an svn update, using the “svn+ssh” protocol. Since it would be silly to leave an unencrypted copy of my super-secret private key on a shared server, I’m now stuck with password authentication. If, however, I enabled “ForwardAgent” in the ssh config on my workstation, ssh uses its built-in tunneling capabilities to create another socket on the dev server that is tunneled back to the ssh-agent socket on my local workstation. This means that the ssh client on the dev server can now send “decrypt this secret message” requests directly back to the ssh-agent running on my workstation, authenticating itself to the svn server without ever having access to my private key.