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.

Some examples where it would be useful: Exposing a zip file or exe-embedded data as a filesystem, or making an FS backed by 9P, WebDAV or SFTP.

Same like mocking databases; yes they make your tests run faster and more “pure”, but you’re suddenly not really testing reality anymore.

Sure, but they're not mutually exclusive approaches. Having tests that run in a couple of seconds thanks to low I/O and cheap scaffolding setup/teardown can be a valuable addition to slower, comprehensive system and integration test suites.

And you can spot differences between your understanding of reality (test mockups) and how it behaves on the real filesystem. (In this case.)

Fault injection is much easier if you can mock IO. And you aren't really testing your software if you're not injecting faults.

My point is, you can give gibberish data to a processing function if it just takes in-memory data instead of being parameterized by “IO”.

If you parameterize everything by IO then you have to mock the IO

It's not just about gibberish data. It's read failures, write failures, open failures, for a variety of reasons including permission errors, disk errors, space errors, etc.

You basically want to test your code does something sensical with either every possible combination of errors, or with some random subset of combinations of errors. This can be automated and can verify the behaviour of your program in the edge cases. It's actually not usually that helpful to only test "we read corrupt garbage from the file" which is the thing you're describing here.