A brick wall is not necessarily considered to be waterproof. Water can penetrate through a masonry crack that is hairline or as small as 1/100th of an inch (0.3 mm). And masonry exterior-covering materials (siding) is porous and can absorb moisture during a weather event.
Weep holes are open gaps along the bottom of a brick veneer wall that allow water to drain away from the wall assembly and out to the exterior of the siding. The weep holes also can provide ventilation in the open space or air gap behind the brick veneer. Ventilation can help dry the interior wall assembly. This ventilated air gap can also help equalize the air pressure and limit moisture movement. Weep holes are usually simple open voids or gaps or spaces in the vertical mortar joint. They could also be made of small plastic tubes. They are typically spaced between 24 inches and 36 inches apart horizontally along the bottom of the brick veneer wall. You may also find weep holes above a header lintel above a window or door. It's common to observe weep holes installed about every 36 inches as a common best practice. www.nachi.org