From what I have seen thus far
every most homes/apartments in Rio de Janeiro are equipped with what can only be described as slave quarters.
The room in question measures somewhere between a refrigerator and a walk-in closet accompanied by an attached bathroom (essentially a shower stall with a toilet and sink inside of it).
I can only speculate at the moment as to the most common uses of these rooms by Brazilians (hiding unwanted relatives?), but it is most probable that these rooms are simply for earning unreported income by way of renting them out by the hour to our prostitute friends in Copacabana (nothing wrong with cheap rent, right?).
But if that's not the case then I am told it is the room where the live-in maid stays – I am unsure if this is a positive or a negative thing for said maid (but most of these rooms have an attached “bathroom” (basically a shower stall with a toilet in it), so that's a plus).
When I first saw the (closed) door to this room I believed it to be a closet (or perhaps where pets were hidden away).
However, many perks do exist to living in such a room.
The quarters in my current apartment allows you to operate the light in the room whilst laying in the bed! All of that hassle of walking across the room or buying a clapper to turn off those pesky lights is eliminated!
Additionally, you can multitask to your imaginations outer limits whilst in the bathroom (you can shit, shower, and shave all at once!).
The beds are admittedly too small to accommodate an adult, but that is a small price to pay for so much convenience.