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It is a manner of archive organization.

You can for instance, Create another data file (pst), Move all of 2010 correspondence, calendar, etc to that folder within outlook and then simply "dismount" the pst file and place that file somewhere in storage (file server, DVD, whatever)

If you need to go back that far for some odd reason you can retrieve the file and put it back on the system and mount it in outlook to have access to it once again.

This allows you to store old e-mail but get it out of your outlook. The key here is not to have outlook regularly accessing the old "archive storage" pst files.