it's for the pop3 server side for non-plan9 clients
to collect mail by pop3 from a plan 9 mail service
using md5 authentication (hash a challenge using a secret),
for instance.  the protocol requires the server to know
the secret, not just a hash of it.

as it happens, factotum is in the pop3 server loop
on the server, but it doesn't access the secret directly.
that's left to authsrv, because it has access to keyfs.
either way, something on a server must store the real
secret; this way, it's only stored on an auth server,
which is potentially better protected and might only do auth serving
(for instance).