They used to need to check for parasites, in the shattered state of misery that existed after the war. (Edit! This is wrong. TIL.) It's disgusting, but so is having parasites and not knowing it.
Pro tip for US people: Get ready! The world is not inherently a safe and stable place, and if you knock out the supports that are keeping it safe and stable for you, all kinds of really bad shit can happen.
Remember the whole "Ivermectin, the dewormer, helps against Covid" thing in the US? Researchers did look at those claims and came to the completely unsurprising conclusion that, unless you have gut parasites, Ivermectin does not help with anything, including Covid. However, if you do have parasites, the Ivermectin can remove a comorbidity, which is obviously going to help with just about anything, including Covid. (Unless you have the wrong kind of parasite, and the sudden presence of rotting worm corpses throughout your body leads to septic shock…)
My takeaway from that is, modern day Americans probably have a lot more gut parasites than we used to think before Covid.
IIRC, this design predates 1945, and is not specifically about parasites but digestive health in general. (There were a lot of health fads in late 19th/early 20th-century Germany, some more quackish than others.)
In a way, this is a tribute to longevity of bathroom porcelain. Even in Germany, this is far from modern. If you look e.g. into the bathroom department of German DIY stores, you won't find this design anymore.
I had worms once and I found out only because of this type of toilet. I must have gotten them from a park where I was sitting down on the grass, without a blanket.
Hookworms can be acquired by bare skin against contaminated soil, so it's theoretically possible, especially if TheBrideWoreCrimson wore their birthday suit