Gasket matching vs. port matching.

I have done the port matching using a template before. The original port mismatch was very minimal in the one engine I did. If I had gasket matched, I would have had to grind away a significant amount of metal.

My question; why grind to the gasket outline?

I've seen dozens of videos of this. Guy sets the gasket to the head and scribes the outline. Grinds away metal to the scribe line. Repeats for the intake.
If you are only trying to correct the mismatched port, what is the point of going all the way to the gasket edge?

Grinding that much metal increases port size in that local area and would slow airflow. Correct?