I was a brakeman on the Penn Central some years ago and we often switched out a couple of pipe yards that were at the bottom of a hill. If it was early morning with dewy tracks, we would sand going down so we could make it back to the top of the hill.

The coal fuels the fire that makes steam from the water, and the sand, when dropped on the rail, gives the engine more traction. Most useful when the rail is wet, covered with snow/ice/leaves. Sand actually is essential to get over the railroad.

Legend has it an old B&ORR helper engineer dreamed up the idea circa 1840s, and rigged a box up on his locomotive with pipes to the rail. It worked so well the RR's quickly adopted it. He shoulda patented his invention. The newest computerized locos today still have sand boxes with pipes to the rails.