Takes the place of lube to prevent leading.
Edit to add: and it looks really cool!
So, why not just shoot copper jacketed bullets exclusively?
Cast is more fun and may in some cased out preform jacketed. The picture below shows what soft lead 280 gr powder coated bullets will do at around 1050 fps out of a 5 1/2" 45 Colt barrel. Weighed after recovery shows less that 12 gr loss of bullet weight.
After 90 some rounds there was no leading and the barrel looked new after one swipe with a wet brush and two dry patches.