Probably to keep contaminents out of the painted surface (skin, oils, hairs). Once the part is mostly under... it doesn't matter if your forearm sheds onto the water. Or simply to make cleanup easier... keep the paint/print out of your fingers/nails/etc.
You can do a swirl type design with just a bucket of water and a certain type of paint. Obviously much more limited.... but you could work it into a cool design. I would think camo like pattern would be easy enough with some practice. Just clear coat afterwards and it's just as good as the printed graphics. Not for hunting IMO, but look pretty sweet.