historically this is what we called "chambre de bonne" (servants room). It used to not include showers or toilets, which could be found in the corridors ( one for all the rooms on the floor). In the 60s and 70s it became more and more used by students that could still go to their parent's appartment. Then in the 90s it became rearranged more and more to be used as a regular appartment with all the equipment.

And during all those years, prices have only been rising. This video shows a flat that probably costs more than 500€ a month ( judging by the look of the street, it.s probably in an expensive neighbourhood).