SCNPlane y position

I’m just getting started with the ARKit by tutorials book and one thing I’m not clear on - when they attach the SCNPlane to the ARPlaneAnchor in the Detecting Surfaces chapter (Page 111), they do

planeNode.position = SCNVector3Make(planeAchor.center.x, 0, planeAchor.center.z)

Why is the y coordinate 0? Why not use the y coordinate of the anchor?

@chrislanguage Can you please help with this when you get a chance? Thank you - much appreciated! :]