In the old style paint the paint would separate and the denser materials would settle to the bottom. When you went to shoot the paint, the balls would fly wildly because they were heavier on one side. By tumbling the paint it kept the paint mixture well mixed and the paintballs flew true. So true I had a red-dot sight mounted on my Phantom. If that dot was ever on you, you were out.
I used a 8x10 photography tumbler for my paint.