I know that they have them in germany. when i was visitnig my grandparents there (now deseased) they had a shelf in the toilet as well. And seen it dozens of times in germany.

They said it to inspect the excrements - and so to detect anything which isnt normal, so it can get fixed. Just a healthy check up on the bodily functions.

That's what they say in germany, I'd assume it's true for NL as well.