Object deletion protection - "Delete child" ACE on parent?

So I know what "protect accidental deletion" flag do, I also know it adds an ACE deny everyone on "delete" and "delete tree". It all makes sense. But what I don't know is it also adds a deny "Delete child" on parent object. From the name of it, it sounds like when you protect a node, all sibling node will be protected as well, but it's not the case. Now the question, why does the process need to add that "delete child" entry on parent object? What does it do?