Departmental ring groups allow multiple people in a department to receive incoming calls at the same time. If a customer was trying to contact the sales department you could have any number of phones ring at once, and whoever picks up the phone first will answer the call. This feature is useful when you have multiple people who normally handle incoming calls for a department and can be used over any geographical distance.
You might have employees working from home and at the office, and either group can answer incoming calls when someone calls the sales department. Call announcement is commonly used with this feature so that employees know what department the caller is trying to reach before they answer the call. This feature is a function of your auto-attendant.