In "The Black Cat", why do the narrator and his wife go down in the cellar?

Of his wife, the
narrator says,

One day she accompanied me, upon some
household errand, into the cellar of the old building which our poverty compelled us to
inhabit.

Thus, we do not know exactly why the speaker and
his wife go down into their basement, but we know that it has to do with a household errand of
some sort. After the fire destroyed their first home, they were compelled to move into an older
buildingperhaps an apartment buildingwith a cellar. Perhaps they went down to retrieve some
household object they had stored in the basement; or perhaps they can food and the narrator was
going down to retrieve some of it. There are any number of reasons that they might have gone
down into the cellar, but it sounds as though it was perfectly mundane.

It is
in this cellar where the narrator murders and then disposes of his wife. He decides to hide her
body within the wall, as he's heard monks of the Middle Ages used to do. The walls were
"loosely constructed" and the plaster had never dried completely, so he is easily able
to dislodge the bricks and then repair the mess with new plaster.