The Construction Plane can be used in certain tools to set the initial orientation and/or position used by the Modeler tools. These tools typically check if the plane is displayed when activated; if it is, they use its transformation and then hide it.
You can quickly set up the Construction Plane using the standard Volatile C-Plane Operation, activated with the "/" hotkey.
You can also activate and configure it in the right vertical panel of the viewer.
One way to pre transform primitives during creation is using the Construction Plane.
In the Flatten tool, the Construction Plane is used for positioning and the direction of flattening.
In the Radial Array tool, determines the position and orientation of the circle.