Forest_LegacyExchangeDN - ProxyAddresses of AD

MIIS 2003 ensures that the ProxyAddresses attribute of Active Directory user objects contains all values of the mulitivalued Forest_LegacyExchangeDN attribute where Forest is not the same as the forest from which the user account is coming. What is the reason for this configuration?
When accounts migrate between exchange orgs and get a new legacyExchangeDN, storing the history of all legDNs in the proxyAddresses attribute ensures that they can still be recognized by outlook with the old legdn.