Common causes of failure
Chimney pots crack from frost damage, age, or a lightning strike, and a missing pot is sometimes simply blown off in a storm — either way it leaves the flue opening exposed.
Why it's worth fixing promptly
An open or damaged pot lets rainwater run straight down into the flue, which can lead to damp problems inside the chimney breast and, eventually, staining on ceilings and walls below.
Like-for-like or upgrade
Replacement is a good opportunity to move to a better-suited pot if the original wasn't right for the flue, or simply to match the existing style if the chimney is otherwise in good condition.