Why does this trap invalid Message-IS's?

Can someone explain (either via the list, or via email reply) why this
recipe traps invalid message ID's?  This recipe is rather bewildering to
me.  I can't tell from the log file if really deletes spam.  But I have
deleted only two genuine email messages in two years, so it must be benign.


# This will allegedly trap invalid message ID's
:0
* !^Message-Id:[\t ]+<("[^"]+"|[^ <>@]+)@[^<>]*>$
/dev/null