Jamieson, Fausset and Brown on Joshua 4:20-24

20-24. those twelve stones, which they took out of Jordan, did Joshua pitch in Gilgal--Probably to render them more conspicuous, they might be raised on a foundation of earth or turf. The pile was designed to serve a double purpose--that of impressing the heathen with a sense of the omnipotence of God, while at the same time it would teach an important lesson in religion to the young and rising Israelites in after ages.