There are two types of archiving, compliance and operational. Compliance archiving is used when you have a legal obligation to maintain strict retrievable email record keeping. Selected email (selected by user(s), domain, channel, incoming, outgoing and so on) coming into the MTA is copied to the archive system before being delivered to the message store or the internet. Archiving can be set to occur either before or after spam and virus filtering.
Operational archiving is used for mail management purposes. For example:
To reduce storage usage on the Messaging Server message store by moving less used (older) messages to an archiving system which uses lower cost storage.
As an alternative for data backup.
Note that compliance and operational archiving are not exclusive. That is, you can set up your system so that it does both compliance and operational archiving.