GSM technology is found almost exclusively in cell phones, so why would you ever find it in a doorbell? So your front door can call you when someone rings it, obviously. That way you can open the door, via a PIN code you enter into you cell phone, whether you're home or not. The Waleli GSM Doorbell would also come in handy the next time you lock yourself out.
Just think, when UPS comes by to drop off a package while you're on vacation, you can let the driver in after you chat him up through the doorbell's intercom. What would be really cool is if this doorbell could play custom MP3s as well.