Unless brass is very dirty (such as being retrieved from a mud hole) any flaws, cracks, potential case head separations, etc. should be readily visible even before cleaning.
Yeah, but it's a lot easier to see when the brass is shiny clean. Especially for us old(er) farts who don't have 20/20 any longer. Inspection time is cut way down for me and any imperfection is much more readily apparent with shiny brass. Especially with .357 mag because of the slender and deep casing. Priming is so much easier as well when your primer pockets are clean.