The object is ..
To get rid of poisonous flue gases safely without them coming in through a vent, window, or door.
Flues are all fanned now .. most being twin wall (exhaust in centre cooled and quietened by intake in the annulus) so the draught issue that used to be .. is no more.
Having said this .. an open wood-burner or older fanned conventional flue would change to twin-wall once the pipe rises through the first combustible wall/floor.