By not removing the dirt and grime from a wall, the chances of any coating you apply failing sooner than it should greatly increases as there is a lack of hold on the surface.
Power washers takes off dust, chalk, mildew, and even old paint using highly pressurized water. They’re not difficult appliances to operate, but they can be dangerous in the hands of an unskilled user. In addition, experienced users know how to remove undesirable layers of dirt without damaging the surface itself.