The purpose of the Grass Bag or Grass Collection System is to collect the grass clippings, bits of leaves, and other small bits of debris propelled into it from the mower deck during operation. The typical grass bag for a walk-behind mower is a cloth bag wrapped around a metal frame. The bag is usually sold separately as it is far more likely to wear out before the metal frame. For larger zero-turn mowers, the grass bag is usually a plastic container that resembles a large trash can.