Sand and grit on the outside of a case can scratch the dies, and over the years that does matter. Also, when the brass is super shiny it is easier to see on the ground and distinguish it from all the not-shiny brass that isn't mine.

When I obtain new brass from wherever, I run it through the wet tumble once to get it super shiny and take all the discoloration off. After that, I just dry tumble it with the Dillon polishing stuff in corn cob media.