Originally Posted by Aeroplane

I've seen them used for folks who have short-short legs compared to their torso, and they can't afford a custom frame. It effectively lengthens the top-tube, without making your stem ridiculously long.

I've also seen them used as a "forward" seatpost (flipped around so they are bent forward) to give a road bike more of a tri-bike style geometry. Makes using aerobars a lot easier because you aren't bent over as much.