Currently, hostfs translates VFS operations into libc file operations on the host. However, there are other possibilities. The userspace side of hostfs could just as easily be operating on a different machine or on a totally different type of data.
It would be straightforward to put a network link between the kernel and usermode pieces of hostfs, allowing UML to directly mount remote filesystems. This would be the equivalent of the usermode nfs server that currently exists on Linux.
The userspace piece of hostfs could easily be used to mount something other than a filesystem inside UML. For example, it could be used to mount a SQL database as a filesystem inside UML.
hostfs could also be used to mount multiple external resources on the same UML mount point. An example of this would be to mount a number of nearly identical external filesystems inside UML and install software onto all of them simultaneously.