If you are in an area covered with WiFi, such as a Starbucks, McDonalds, Home, etc, you surf the web at higher speeds than even 3G. Also, if you have poor cell signal at a covered location, you will still be able to get your emails and surf the web.
My old boss had zero cell signal at home, but he had WiFi, so he could still use his BlackBerry at his house (minus Voice)
I like having it in my office because I can send pictures to myself a lot quicker from the BlackBerry than waiting for it to send over cell network. Im too lazy to connect or use Bluetooth to transfer files