In case of any commotion outside your main door, you can activate the camera installed outside and check what is happening on your Monitor.
You can check who has rung your door bell without going upto the main door.
You can speak to the visitors through the video door phone before deciding to open the door
You can avoid opening the door to strangers. People with poor eyesight can check visitors on a large screen. In case of darkness in the corridor/outside your main door, the LED lighting around the camera lens, makes it easy to view the visitor
For kids who cannot reach the peephole/ magic eye, can check who is at the door through the monitor of the video door phone