Q. Why do some people leave a section of the wall in their house bare?
The idea is to leave bare a square cubit of the inside wall facing the door to recall the destruction of the Temple. In the past this section was painted black but today that might be self-defeating, as black is sometimes part of the internal décor. The rule about leaving a section bare applies if the house was built by a Jew for Jewish occupants.
Magen Avraham (Orach Chayyim 560:1) says that if a house bought or rented from Jews does not have an undecorated square one should peel off one to expose the brickwork.