A public toilet room is called a restroom because it's a place to refresh yourself and get relief from needing to pee and poop.
It's a much better way of saying public toilet and most people prefer to just say restroom because it's seems more polite.
The word restroom has been in use since 1900 so it's been around for quite awhile and I guess you would take a break and rest while relieving yourself in the restroom.
Although I would not want to just sit in the restroom and rest.
Restroom would really be a better word for a room where people can sit and rest and not use the toilet.
The word restroom for bathrooms or toilet rooms has always baffled me and I wondered why they would call it that.
But that's what I've read online.