Cached Exchange Mode is the process of when 2003/2007/2010 downloading a copy of the users mailbox and storing it locally on their workstation. This means all emails opened by the user from there onwards does not hit the Exchange servers significantly reducing load.
Many clients however still disabled cached Exchange mode on their users workstations. When asking them "why", their answer is always:
Because we need the user can access the most updated address book when they click the global address book.
If your company has this requirement this doesn't mean you need to disable Cached Exchange Mode. You can configure a registry key on your clients to simply keep the address book in online mode.
For more information on this key for Outlook 2003/2007/2010 please see:
Generally the only time you want to disable cached exchange mode is if your users run on a terminal services or Citrix shared environment where you do not want a copy of EVERY users mailbox downloaded and stored locally on the terminal server!