The build-up of deposits in the chimney can lead to a soot fall which can block the chimney. If the chimney is blocked smoke will very likely leak into the house in great quantities. This is dangerous especially if this happens when you are asleep.
Soot builds up in a chimney over time, especially when burning wood or bituminous coal and especially when slow burning overnight. From time to time the soot becomes dislodged or grows to such a size that it falls down. A soot fall is accumulative meaning that it ‘snowballs’: if the soot fall starts at the top of the chimney then it can bring down much of the soot below it as well. This can block the chimney or stove flue pipe. The stove can then smoke a lot on a far grander scale than the wisps of smoke alerting the user to the fact that the flue or chimney needs sweeping.