Here are my thoughts. It is useful to have a rack in the shower to dry of articles of clothing that are dripping wet like bathing suits. Also, many times in small bathrooms there is only one towel rack, but if two people share the bathroom, and additional rack in the shower can provide a second place to allow a second towel dry during the day. Also, if the towel rack in the bathroom is for the pretty towels, the bath towels can be stored in the shower, even if you pull them out during the shower. Some apartments have only one bathroom, so this would keep the grubby paws of visitors off your bath towels.
If the shower is long enough, the towel probably will not get very wet during a quick shower.
Baths way back in the day were more common, people shared baths, you know the old saying don’t throw the baby out wi the bath water, the poor baby went last in the dirtiest of water. So, I would guess poor people probably took more baths than showers, just a guess. Baths use less water if multiple people use the same water. Also, water pressure was not the same, so showers were not as good as they are today. But, by the 50’s most buildings and houses in the US had adequate pressure and by the 50’s people would have found it icky to share baths probably like we would today.
When I was little, in the early 70’s, there was a push to conserve water, and it was touted that a short shower uses less water, but a bath uses less than a long shower, so people could choose I guess which was best in regards to water usage if they cared.
When I was in England back in the mid 80’s a couple hotels I stayed in had only a bath, no shower. I would guess that many Europeans who emigrated to America were accustomed to taking baths.