surface. If you go buy a brand new Lodge skillet, it'll be incredibly rough and feel like 20 grit sandpaper. To get a nice smooth cooking surface on it you need a lot of seasoning to start building up a nice patina (filling in the rough surface area bit by bit) and lots of use/cooking to help smooth the surface out as well.
Grinding down like the guy in the video gives you a nice glass-like surface right off the bat. So you have all the benefits of iron without the horrible, gritty surface of a cast metal.