Routable road segments are connected to each other with junction nodes.
Pedestrian boardwalks and Walking Trails are the only road types which can be connected to other roads without creating such a junction node. When dragged to a drivable road segment, they instead create a virtual node, also known as a phantom node.
Phantom nodes don't work for routing, so if a Walking Trails plays a role in routing the WT should be connected via a real junction node. Pedestrian boardwalks are not used for routing so for them, there is no need to split a segment. They can use phantom nodes.