Actually people have sand beds for a variety of reasons, nitrate removel is one of the motivations for a Deep Sand Bed (DSB). Ofcourse nitrate removal can be accomplished other ways as well, the most common being regular water changes (a good idea for numerous reasons) and the growth and harvesting of plants. None of these are exclusive of the other, I do all three.
In addition to nitrate removal, I was considering what sand offers when stocking the tank. Many critters (fish, shrimp, crabs, conchs, worms, etc) need or prefer a sand bed. Some of the most enjoyable inhabitants in my tank rely on it. I also like the look of it, so appearance played a part as well.
I have a DSB (4+ inches) in one of my tanks and a shallow (1.5 inches) in my other tank. I am quite happy with both.