The main sail cover is intended to protect the main sail from external influences. Since the sails themselves do not have protection against the sun, we need to protect them with other covers when the sails are not in use.
A Lazy bag or Lazy jack is designed for storing the sail when we are not sailing. It facilitates easier lowering and raising of the sails and allows for quicker stowing after sailing. The main purpose, however, is to protect the main sail from weather influences.