It is used to connect two different node groups into one. You can drag a output wire (must be value or object, not call) into the null input to connect it. This can be used to make it easier to pack separated nodes together, because when they are part of the same group you can select one node and it will select them all. It can also be used to work around this bug by forcing the node groups to be rebuilt: https://github.com/Yellow-Dog-Man/Resonite-Issues/issues/907