The bundle is useful for updating the history of repositories on machines where the normal transport mechanisms can't be used due to missing network connections between the machines, firewall rules, and so on. There are, of course, other methods than the Git bundle for transporting history to remote machines. A bare repository on a USB stick could also be used, or even plain patches can be applied to a repository. The advantage of the Git bundle is that you don't have to write the entire history to a bare repository each time you need to update a remote, but only the part of history that is missing.