Many times, the chaff from a threshing floor is thought to be light and worthless and simply blown away by the wind - a natural way to separating the chaff from the wheat. Here, John conveys an imagery of the "chaff" not being blown away, but being burned.

If you think that the consequences of being chaff will be non-existent, or just be getting bounced around by the breeze, John clears it up that God has no time for this and will completely destroy the chaff.