Think of things that count as one set. The clearest modern picture is a door in a bedroom wall: open it and your own bathroom is there, so the two rooms are not two separate places but one unit you have to yourself. Everything on this page is that idea seen from a different angle. Describe the small room and you get an en suite bathroom; describe the big one and you get a bedroom that is en suite; point at either as a thing and en suite becomes a noun. The older use drops the door altogether and keeps only the set: a hat and gloves made en suite belong together because they match, not because they touch.