When I was young, women and girls wore slips under their dresses and skirts. Here is a fine example. The half-slip was also popular and quite acceptable.
They were never supposed to show, but we all knew they were there. In fact, we had a code...if we saw a girl at school whose slip was showing below the hem of her dress, we would tell her, "It's snowing down south." She would immediately head for the girls' bathroom and adjust the garment so it was out of sight.
We were trained from an early age that this extra layer was necessary.
They served as modesty protectors, in case one were to find oneself backlit, like Diana Spencer, in this photo that was taken before she became a Princess:
I saw a really good play yesterday, and I was asked what I thought of the costumes. The worst thing I noticed was that the lead actress was not wearing a slip, and the play was set in the 1940s. Considering some of the costuming I've seen on stage, that's pretty minor, but still.
It's attention to details that separates the sheep from the goats, IMO.