John Calvin Commentary Exodus 40:38

"For the cloud of Jehovah was upon the tabernacle by day, and there was fire therein by night, in the sight of all the house of Israel, throughout all their journeys." — Exodus 40:38 (ASV)

For the cloud of the Lord was on the tabernacle. Moses more distinctly explains what he had said generally respecting the cloud, namely, that by night a fiery column appeared, because the cloud would not have been visible amid the darkness. A second explanation is also added, that this did not happen once or twice only, but in all their journeys; so that they were never without a sight of the cloud, which might be a witness of God’s presence, whether, being settled on the tabernacle, it commanded them to rest, or, by its ascension, gave them the sign for removing the camp.

Now, the consistency of this process, in all the variety of times and marches, contributed in no small measure to their certainty; for if the cloud had daily followed the same course, this very regularity would have obscured the power of God. But when for a whole year it did not move, and then frequently proceeded to a new place, and now by its guidance pointed out a longer journey, now a shorter one, by this very diversity the paternal care of God, who was never unmindful of His people, was more clearly revealed.