Why did every house in Harappan cities need to have at least one wall along a street?

To ensure proper flow of domestic wastewater into the street drains

The Harappan cities were characterized by their remarkable drainage systems, which were meticulously planned. When examining the layout of the Lower Town, one can observe that the roads and streets followed a precise "grid" pattern, intersecting at right angles. Evidently, the streets with drains were established prior to the construction of houses. This arrangement ensured that domestic wastewater could flow into the street drains efficiently. Consequently, each house was required to have at least one wall along a street to facilitate the proper functioning of the drainage system.