A video intercom is a type of door phone that has a camera on the outdoor station and a display of some kind on the indoor monitor.
Then someone presses the call button on your video door phone it activates its camera and alerts the indoor monitor. You then talk to them while you view their image. You can even press an open lock button if you have an optional electric door lock installed.
There are lots of good reasons to get a video door intercom. If you've ever had your kids run to the door and open it to a stranger, you know one reason to get a video door phone. Kids love toys and they'll see the video door phone as another toy they can't wait to play with. So instead of running to the door, they'll run to the door phone monitor. That at least gives you time to make sure they don't open the door to a stranger.
For children who are old enough to be at home by themselves, and for you, a video door phone also enables them and you to conveniently and safely see who's at the door without opening it, or even getting anywhere near it.
Another great benefit is that you don't have to open the door to solicitors. It's so much easier to say "no thanks" to a good salesperson when they can't even see you!