(let us disregard the master bathroom for the moment)
The bathroom is simultaneously the most public and private room in the house. It’s where we accomplish our most intimate personal activities, barring lovemaking—though that happens there sometimes as well. At the same time, everyone uses it.
In western culture, this paradox has led to the development of the water-closet and the master bath. It is a conceit of affluence. But in most middle/lower-middle class houses, especially pre-1990s, there is one bathroom.
Here we find some decoration usually. And storage. Most everyone’s bathroom holds the same stuff—the necessities of grooming and medication/first aid (the bathroom is where you end up when sick or hurt due to clean water, good lighting, and a toilet).
The bathroom is, more than anything, a statement about what all human beings share in common, but would rather we didn’t have to. Bathrooms rarely contain secrets we don’t all know already.