This dude claims that the rough surface is important for the pan being able to hold the seasoning.

He does a test: seasons two pans, one smooth and one pitted. Cooks, then cleans. The seasoning didn't adhere well to the smooth pan.

"okay so let's take a look at how the seasoning performed.
and on the smooth surface pan we can see little areas where the seasoning was lifted off"

https://www.youtube.com/watch?v=WFhRfbiLpJ8