A "mulch plug" or "mulching plate" is only necessary if the operator would like to mulch the lawn during mowing. This plug/plate is used to close the discharge opening in the mowing deck and recycle the lawn clippings back into the blade area for repeated cutting.

Note: Most, but not all modern residential style mowers can perform the mulching task. A mulching blade, if kept sharp and balanced, should also operate efficiently when used to discharge or bag grass.