I know it's obvious, but it must be completely different to walk around with no bra in the house as it is outside....social customs just don't really apply inside the house.
Other than the "practicalities" to hide errant erect nipples, I would say that it's just western social values and pressure. Women are pressured with this weird expectation of the "perfect boobs" and stuff, which leads to the suggestion that a bra is desirable to keep them shapely.
Female nipples also have some sort of weird taboo in western society, as well, with the link made to lactation and breastfeeding...even in women who have not had a child.