The lid is intended for use with the Auk Mini planter system. The planter only supports 1-2 plants when growing peppers or tomatoes, meaning you have to leave the remaining slots uncovered or put the empty boxes in them. I found that leaving the empty boxes in the system causes nutrient waste to build and crust up when the water level is high enough. Replacing the boxes with a flat lid on top prevents this as the water level won't reach.

The lid isn't completely level with the base of the planter boxes, but my prints slot in perfectly in the frame.