Swiss water troughs have two compartments, one large one small - why?

Question

I asked this question a week ago and got two answerrs both saying it was a precaution against freezing. This is nonsense, for if one compartment freezes both freeze. Water comes from a stand pipe or other source into the large compartment and flows into the smaller one through a cut in the separating wall. The smaller comartment has an overflow device of one sort or another. Both compartments can be completely drained by removing the plug at the base of the compartment.

Answer 1

one to wash , the other to rinse .