Charles Ellicott 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)

The Lord went before them. —In Exodus 13:17-18, the writer has declared that God led the people; he now explains how. From Succoth certainly, and probably from Rameses, He moved in front of the host in the form of a pillar, which had the appearance of smoke by day and of fire by night. The Israelites marched, it is implied, some part of each day and some part of each night. This would be in accordance with modern practice and is an arrangement introduced to ensure the march is accomplished before the sun attains its full power.

The pillar was at once a signal and a guide. When it moved, the people moved; when it stopped, they encamped (Exodus 40:36–38); where it went, they followed. It bore some resemblance to the fire and smoke signals which generals used when at the head of their armies (Lepsius, Denkmäler, vol. ii., pl. 150, 2; Papyr. Anastas, 1; Q. Curt, Vit. Alex. v. 2, etc.), and indicated that God had constituted Himself the generalissimo of the host; but it was altogether of a miraculous and abnormal character.

To go by day and night. —The night journeys of the people are mentioned again in Numbers 9:21.