A lot of the extended chokes are for extremely tight chokes or for steel shot. The idea is that the choke doesn't start it's constriction until it is past your barrel. That way, any damage that is done is done only to the choke, which is cheap and replaceable, rather than damaging your barrel. Again, this makes most sense when it is on an extra-super-stupid-ridiculous-tight turkey choke, or for use with steel shot, which doesn't compress. For extended tubes in normal restrictions, they may opt to spread the constriction over the entire length of the tube, thereby making it more gradual, and therefore less damaging to the shot.