How are etc/*.map files used?

[Eli Zaretskii]

The file etc/charsets/README says nothing about the intended usage of
the *.map files in there, it just tells how to generate those files.
Are we using these files somehow, and if so, where?

I bumped into an email today that used IBM037 charset, which I
couldn't read until I manually made a coding-system-alias for it from
EBCDIC (which isn't quite right).  But etc/charsets/IBM037.map does
exists, so it sounds like Emacs ought to support that encoding.

What am I missing?