Portable showers are kind of silly, I’ll grant that. You fill a big plastic bag with water and set it in the sun. When the water heats up enough (to 105°F or so), you hang the bag from a tree, stand underneath it, open a small plastic valve, and voila, you have a shower au naturel. Ahhh. . .
You can use one near your swimming pool to rinse the chlorine off after a swim. You can use them while camping. You can use one for a hot water bottle big enough to soothe your entire back or stomach.