A couple of things:
1. Offer a longer choke tube, so the constriction happens over a longer period of time which doesn't damage the shot as much. This is similar to having your forcing cones done.
2. Extended tubes are often ported which allows for less muzzle rise and a faster/more controlled follow up shot
3. THEORETICALLY causes less wear on the barrels due to point 1. In practice it won't make a difference.
4. Extended tubes can be changed on the field without tubes because you can grip them.
5. You can have your 28" gun swing with the dynamics of a 30" gun. Or a 30 to a 31 or 32".
6. Easier to clean off in the field should you accidently dip the muzzle in the sand or mud.
That's all i can really think of. I'd place most value on point 2 personally.
Are they worth buying: yes and no. If you see any value to any of the points above and that value is equal to or greater then the cost of the tubes then yes. Will it help you hit more targets or destroy them better if you do - no.
I'm buying two half chokes, extended and ported but that's for the recoil issue, not for anything else. I'm hoping they'll pattern decently.