If you think about it, most usages of iBeacons won't always involve the app running in the foreground. Most apps are most likely to be awakened when entering the boundaries of a beacon region and then brought into the foreground by the user if they want to use the app.
Imagine that you're building an app for retail loyalty. In this scenario, you are almost certainly going to want to trigger some functionality when the user comes into the range of your store. You might want to send the customer a tasty offer for your new bagel range, or simply offer users loyalty cards just in case they want to pop in for coffee.
