Prevents the static cling, so that when your cartridges are loaded, and you finish tumble loaded rounds, the powder is harmoniously balanced and suspended concentrically directly in front of the primer.
YMMV
Also, suspends fine dust and dirt in your tired media...making it appear cleaner, and thus more useful.
In actuality, when its that dark, most corn media types are much more round than acute on the granules, decreasing efficiency. Gently scrubbing away surface stains is much easier than burnishing them away.
Basically, go with what RC said if you don't use a two-step media process.