Albert Barnes Commentary Exodus 13:21

"And Jehovah went before them by day in a pillar of cloud, to lead them the way, and by night in a pillar of fire, to give them light, that they might go by day and by night:" — Exodus 13:21 (ASV)

Pillar of cloud - The Lord Himself did for the Israelites by preternatural means what armies were obliged to do for themselves by natural agents. The Persians and Greeks used fire and smoke as signals in their marches, and in a well-known papyrus, the commander of an Egyptian expedition is called “A flame in the darkness at the head of his soldiers.” Thus, by this sign of the pillar of cloud, the Lord showed Himself as their leader and general (Exodus 15:3), (Exodus 15:6).