Just plain old curiosity.
In the same way that we use an American-style toilet, a Roman user would sit down, take care of business, and watch number two float blissfully away down the sewer system. But instead of reaching for a roll of toilet paper, an ancient Roman would often grab a tersorium (or, in my technical terms, a “toilet brush for your butt”). A tersorium is an ingenious little device made by attaching a natural sponge (from the Mediterranean Sea, of course) to the end of a stick. Our ancient Roman would simply wipe him- or herself, rinse the tersorium in whatever was available (running water and/or a bucket of vinegar or salt water), and leave it for the next person to use. That’s right, it was a shared butt cleaner. (And of course, there were other means of wiping as well, such as the use of abrasive ceramic discs called pessoi.)