It's not always about mocking (in my cases it hasn't been). Sometimes it is about multiple "real" implementations - a filesystem is itself an abstraction, and a very common one, it seems like it would at least sometimes be useful to be able to leverage that more flexibly.
I have a little system that takes a .git and mounts it as a fuse filesystem. Every commit becomes a directory with a snapshot of the project at that point in time.
You could read the whole .git in at once, and then you'd have an in-memory file-system, if you wanted to.
In any case, I agree with you: it's not about mocking.
