Why ("/#[^/]+#\\'" . emacs-mule) ?

[Per Starbäck]

Why are #autosave# files opened with the emacs-mule coding?

I don't use recover-file a lot. When I have to use autosave files I
usually open them up in their own buffers and compare and copy
"manually".
Very often when I open them I have to immediately use
revert-buffer-with-coding-system because they are utf8 files.

For a long time I have wished that the coding of those files would be
recognised like for other files. I thought of reporting this as a bug,
but since
auto-coding-list (in lisp/international/mule.el) has an explicit line

    ("/#[^/]+#\\'" . emacs-mule)))

it's evidently on purpose, so instead I will ask here. What is that purpose?