This would have been used to kneel in when scrubbing the washing using the vintage wooden washboards. 
Kneeling inside gave the knees and legs some protection from the water. The board would have been placed resting on the front edge then down into the water and the washing would have been scrubbed up and down the board. 
These are fantastic display shelves when hung on the wall. 
Measurements: 49 x 17 x 61cm high