Chimney caps protect the chimney. They not only prevent rain from falling straight down into the flue system, they also keep debris out. Water that falls into the chimney will increase the rate that creosote glazes and tars. This makes removing creosote that much harder. Similarly, any debris brought into the chimney by birds or raccoons is extremely dangerous. This can catch fire up in the middle of the chimney, causing a chimney fire which can really damage the chimney, roof, and even burn the house down.