Why would a Node's parent's point be Rc rather than Box?

[eyalroz]

In https://github.com/nrc/r4cppp/blob/master/data-types.md

an example is given of a Node class which refers to a parent node. It should be explained why that field is Option<Rc<Node>> rather than Option<Box<Node>>. Or - that it doesn't really matter.