Caps help keep the rain out which extends chumney flu life, no animals, birds, and also keeps down the smell caused by rain soaking into soot and creosote. I burn wood, 3 to 5 cords on average and since I put the cap on thier has been a noticable difference with the draft and smell.
Swimmer a.k.a. YO YO MA
Serial Mailbox Killer/Seal Fisherman