Being able to reverse the direction of the lap can be helpful when polishing. Sometimes the crystal structure of a stone being cut ends up being oriented in a direction on a facet where it polishes better in one direct than it does in another. The best analogy I can think of to compare this to would be wood grain. Wood cuts, planes, and sands much better with the grain than it does against the grain.
As for teaching beginners faceting – IMO keep it simple and inexpensive. Let them first learn to cut quartz or colored glass. 260, 600, 1200 laps or toppers, and a master lap to use with Spectra Ultra-Laps or Marsh Howard’s LL-Lite-Ceox Topper lap. I wouldn’t buy tin, batt or any other expensive laps to teach beginners on. I’ve seen too many beginners who would damage or ruin polishing laps by missing indexes and cutting into the surface of the lap with a sharp corner of the stone.
Larry Cashatt