Without a cap, rain, snow, and ice can fall directly into the flue, causing damage to interior chimney walls. Over time, cracks form in the chimney lining, possibly compromising the integrity of the whole chimney structure.
Birds and rodents love to nest in chimneys. Stop their access with a properly installed chimney cap before they do real damage to your chimney. Nests clog up flues, causing smoke to back-up into your home and putting you at risk of a chimney fire.
Most modern chimney caps have a screen or mesh that act as a spark arrestor, similar to your fireplace screen that keeps sparks and embers in your fireplace.