The debug window shows the id of the debug node that produced the output, except when it's inside a subflow. In this case it shows the id of the subflow instance (the calling node).
Within a function node, node.id returns the id of the node. If the function is inside a subflow, it would be useful to be able to return the id of the calling node. Along the lines of node.parent.id, where parent was only non-null for subflows. Logically it would also extend to node.parent.parent.id for subflows within subflows. Note that this is different to using the $parent context, which is at the flow level.
My use case is for logging within a subflow that is used in multiple flows, to know which node called the subflow. I have many flows and each calls the subflow many times, so using $parent.name doesn't give me enough information and $parent.id doesn't exist (and would logically be the flow id anyway).