This node simply passes its input through to its output. If this node has an output but no input, it generates an empty stage with an empty active layer.
This can be useful as a way to refer to a point in the network by name. For example, you might want to refer to the final output of a node chain. If you just referred to whatever node was at the end of the chain right now, someone (or you) might come along layer and add more nodes, or delete the last node, or change its name because they changed what it does. Putting a Null at the end of the chain (or wherever you want to be able to reference the output by name) solves these problems.