You don't seem to have actually read the post. But, if you have and just don't understand, there are a lot of different custody situations with children that are possible, and when you throw a hostile ex-spouse in the mix, it's really pretty easy to see why having a way to communicate with your child in that way, if the other person will agree to it, could be very good for the child and parent. It could have something to do with having an opportunity to talk to the kid when he's not actively being monitored; it could have to do with the kid being able to call the other parent if they're being abused. It could just have to do with wanting to be able to stay in touch with your kid when they aren't with you, and have a video call before they go to bed at night.
It's not a perfect solution, and it raises a different set of issues, but in the context of a weird custody situation it's easy to understand why it might be desirable.