Re: definition of messageID?

What is "messageID" for?

 4.6.1.  MESSAGE-ID / RESENT-MESSAGE-ID

         This field contains a unique identifier  (the  local-part
    address  unit)  which  refers to THIS version of THIS message.
    The uniqueness of the message identifier is guaranteed by  the
    host  which  generates  it.  This identifier is intended to be
    machine readable and not necessarily meaningful to humans.   A
    message  identifier pertains to exactly one instantiation of a
    particular message; subsequent revisions to the message should
Standard  for  ARPA Internet  Text  Messages each  receive new
message identifiers.

(According to RFC 822)