A great place to learn a lot about the BlackBerry is the BlackBerry FAQ site at
BlackBerry FAQ - BlackBerryFAQ
Desktop Manager has several wonderful uses. It is used to sync your contacts, notes, calendar and tasks from your PC if you do not have BES (BlackBerry Enterprise Server). It also contains the drivers to adequately charge your BB (those can be installed seperated).
Desktop Manager is also required (for older devices) to load updates of the handheld (device) operating system to the BlackBerry. And you need it to load apps which don't have the ability to be loaded over the air.