Questions about Bathwater and What People Do with It

I think about things. It's just what I do.
So, this morning, in the dark stillness while the rain was pounding on the roof, I thought, "Why would anyone throw a baby out with bathwater?"

I mean, why in any circumstance or situation was that phrase a) necessary to say and then b) popular enough of a said phrase to live on as a life lesson.

Did people actually throw out infants with dirty water at some point in time?

-b