There are some fish that will "sift" sand naturally. Corys, various other catfish and loaches, and many cichlids. Sand is much better than gravel for these fish. They expel the sand through the fills and eat any food they may find in it.
I have been changing my tanks over from fine gravels to play sand. Overall I think this is better. It does depend upon the fish and their requirements of course, as there are some instances where a fine gravel and river rock substrate would be appropriate, as in a Central American themed aquascape, or an Indian mountain river or a Chinese stream for Hillstream Loach.