Leviticus 6:12 - Matthew Poole's English Annotations on the Holy Bible

The fire coming down from heaven, Leviticus 9:24, was to be perpetually preserved, and not suffered to go out, partly that there might be no occasion nor temptation to offer strange fire, nor to mingle their inventions with God's appointments; and partly to teach them whence they were to expect the acceptance of all their sacrifices, even from the Divine mercy and grace, signified by the fire which came down from heaven, which was a usual token of God's favourable acceptance. See Poole on "Genesis 4:4", See Poole Genesis 4:5. Every morning; though the evening also be doubtless intended, as it appears from Leviticus 6:9, and from the nature of the thing; yet the morning is only mentioned, because then the altar was cleansed, and the ashes taken away, and a new fire made. He shall burn thereon, i.e. upon the burnt-offering, which thereby would be sooner consumed, that so way might be made for other sacrifices, which were many.