Basically it automatically moves older items (you can define "old" in the
settings) from the main pst-file or Exchange mailbox to another pst-file.
This will keep the main pst-file or Exchange mailbox smaller. This can be
handy if you are having limited space in your original mailbox or keep your
main pst-file smaller so it is easier and faster for backups (of course you
should run a backup of your archive pst-file as well after AutoArchive has
ran). In some cases, when you have a really big pst-file, it could also
increase Outlook's performance but this is hardly noticable on modern PCs
though.
In addition you can use AutoArchive to automatically delete older items. For
instance, if you have a folder where you you have your newsletters delivered
too, you might want to automatically delete the newsletters after they are
older than 2 weeks when most offers have expired anyway.
Either way, when you decide to use AutoArchive, make sure you also configure
your backup application to backup your archive file as well (at least after
every archiving interval) as your data is now split over 2 files instead of
1.
