I think valve lapping is about being able to see how your machining came out, then adding to the quality of valve sealing.
It's long ago butt I think when I taught valve lapping via multiple angles on the seats that's why we looked at the line made by lapping.
Taught it years ago don't mean I knew squat about it then or remember squat about it now.
All of it was taught per book work info/theory and not from years of experience.
and... ????????????
among many other things and certifications I once had an Ohio teaching certificate to teach Auto Mechanics.
tooooo much junk in my head about mechanical stuff to remember anything ok. ...