It's from an older technique where mostly the only solid white base was where it's white. For instance, to underbase a yellow, in PS, we would convert the yellow to a grayscale, then invert it; yellow converts to a 20% gray, use an 80% base under it; Royal Blue converts to 85%, use a 15% base under it. It's not quite that simple, as adjustments are needed depending on the image, but that's the crux of it. It does work, and makes for a softer hand when the base isn't all solids.
Steve