myblueberry.com as well as a bunch of other companies offer hosted BES/Exchange. This allows you to sync pretty much everything you have in Outlook (mail, calendar, contact, tasks, etc) wirelessly with your handheld. As well it gives you full TCP/IP connectivity with the internet, where-as the carriers often limit the connectivity you have.

For straight up email you most likely don't need BES, but for most games, and applications you may find yourself needing it.