Explanation: The ancient Romans used the xylospongium or tersorium for wiping purposes - a sea sponge on a stick. How lovely! In public restrooms, they would have been shared, which is probably not all that hygienic regardless of the fact that they rinsed the sponge after each use, or let it soak in vinegar or salt water when idle. Still, civilization! What marvels, right?
It's funny where observation leads one - the Romans didn't understand why salt water was good for sanitizing (that most microbes are not halophilic), but recognized by practice that it was good. There's a bit in De Architectura where Vitruvius discusses how a town dug a channel from the sea to turn the nearby swamps into a brackish marsh, and that disease rates plummeted afterwards. If memory serves he attributes it to 'poisonous vapors' being removed by the saline water, while we would recognize it as destroying the ideal environment for mosquitos and other such insects.
And they used to wash by scraping themselves and covering their body in olive oil.
I remember reading about Romans laughing at "barbarian" foreigners from Gaul who would create bars out of animal fat, ashes and flowers to wash themselves with, like a bunch of heathens.
Makes much more sense that the use it like we do nowadays with toilet brushes. Especially if there was a mural telling people to use it. Usually you don't have to reminder or care to tell people to wipe their own ass.