The public folder is a shared space. It is a very good way to organize or consolidate data, lists, etc. that is used by multiple people that have an exchange account. One example is a shared data list. Different users from the same department can share a task list. Each of them can work on tasks off the list and update them accordingly. This way, everyone in the department knows what's going on, and the items can be viewed, edited, created, and completed in one central location.
Hope this helps!
In addtion, the public folders can be assigned owners who can then control the permissions for who can view and edit the folder. An owner can set the default view to invisible with no permissions and then grant permissions only to those he wants to see and work with that folder. However the Exchange Admin can modifiy these permissions as well using a tool called PFDAVAdmin even if the folder is not visible to him on the folder list in Outlook. This tool also allows the Exchange Admin to perform bulk operations such as replicating all folders to another Exchange server. Many folders in our organization are calendars, but there are also contacts and mail item folders as well.