It's a great question, because many organizations believe backup and archiving. are the same process. If you're currently using backup, the risk is what happens when you have to access emails for discovery purposes, or if an employee wants to go back and reference an email from a while ago. Those are hard tasks and can be expensive tasks if all the emails are on backup tapes. There's also risk that you can never get the data backup...
from a backup tape – it's either corrupted or you can't read the tape. So there's a risk in accessibility. Our suggestion is you always want to have backups in case of a restore, but archiving should keep data more accessible and searchable in the event that somebody actually needs to use or retrieve messages rather quickly.
Go back to the beginning of the Email Archiving in 2008 FAQ Guide.