Eventually the Line in 3D is defined in two clicks and not by three. First click will pick both the plane is drawn to and the point in 3D. The second will be drawn on the same plane. This was done by unifying two pipes in one picking plane and 3D point. Also, it gets notification when to stop the plane searching and also it can get reset.
This can make things easier in 3D to pick the center on a shape without any other user intervention. as is natural that it is intended to make the work on that face.
So in the past there was a graph that Line3D require 2 inputs: a PlanePicking and a Mouse3D coordinate. The downside was that Line3D was need to manage the relations. Unifying them in one common pipe makes the code a bit more clearer and also the Line3D skip managing the relations and to help managing both. The PlanePicking can be queried to get the plane you are in... so it may make things in the same way as having individual PlanePicking and Mouse3D.
Also, it makes possible to not make a bit of order of events housekeeping as the pipe will make clean what means the click (only a 3D coordinate for the plane under the mouse).