For a 9720, you'd need BIS to access anything data-related when not on WiFi (plus certain services like BBM). Without it, and without WiFi, you have phone calls and SMS.
BIS covers overall internet supported stuffs like, BBM, Email, Whatsapp, Browser, BlackBerry World etc, if u have no BIS but only wifi, then except BBM all of the above things will work. Without BIS and Wifi, only regular calls and sms will work.
Hope this will help u.