Novelty. It's a toy, I can't think of any specific purpose that a handgun couldn't fulfill. Now if they made one that was a semi-auto and held an 8+ round mag clip, that could be useful in a certain situation.
I purchased up a Super Shorty and love the contraption. Three shots and one hell of a wide pattern be it buckshot or bird shot. For a SMALL shotgun to clear a hall you can't go wrong. Not an overbore pistol by any stretch of the imagination but a different solution.
A buddy purchased one with the breacher barrel for removing locks adding a new use. Myself and another partner bought one each as soon as we had our hands on them. Not to mention its only a 5$ tax stamp being an AOW (any other weapon).