The purpose of a washing machine outlet box is to allow the washing machine valves for water to be recessed within the wall and act as a central point for connecting a washing machine's water supply hoses and drain hoses.
The washing machine outlet box provides a neat and organized way of managing the drain hose and water hoses for the washing machine and also offers easy access for any maintenance that may be needed.
The washing machine outlet box also makes for a clean and nice look.
You can install washing machine drain lines and water lines and valves on the outside of the wall without needing to install a washing machine outlet box.
But the washing machine outlet box makes it look better.
And some washing machine outlet boxes help to ensure compliance with plumbing codes by providing a standard location for any valves and backflow prevention.
The washing machine outlet box also eliminates the need for a separate laundry sink for draining and it can also help to free up some valuable floor space.
And by having the washing machine water lines, drain line and valves recessed into the wall with the washing machine outlet box it allows you to push the washing machine further back towards the wall.