Much the same as for architecture.
T/S helps prevent distortion at wide angles.
So if you were shooting with your 10-20 set to 10 (16mm equiv) the edges would potentially distort
The T/S helps correct this. PS will also correct it, but you lose some of the photo.
T/S lens also help change the perspective of the shot, and can alter DOF also....(sorry knowledge just ran out)
IMHO they are great lenses, but quite time consuming, and very expensive. There are literally millions of stunning photos on the net that were taken without a T/S lens. I would steer clear unless you are feeling well off! Spend you hard saved cash on a lens you will use more often
(also - not sure whether the t/s effect is lessened on a crop body...anyone?)