The purpose of your chimney crown is to keep rain water, leaves, animals out of your chimney. Having a mesh wrapped around your chimney cap will act as a spark arrester. Having your crown sealed with elastomeric coating will prevent freeze thaw damage which is caused by water getting into your small cracks around your crown and freezing. When the weather warms, the water then thaws and causes expansion and contraction. Once this happens the crown will need to be repaired or totally replaced.