They are not functionally equivalent. A "bzr branch" with a shared repository will only populate the working tree and does not have to duplicate the repository. It is functionally closer to "git-new-workdir" than "git clone" or "git branch".

To have instant branching in Bazaar, you can use co-located branches.

Branches with their own directories exist for the use case where the accumulated cost for rebuilds after branch switches is more costly than populating a new directory with a checkout or if you need two checkouts in parallel. They also exist in Bazaar for simulating a workflow with a central server and no local repository data.