C. F. Keil and Franz Delitzsch on Leviticus 6:13

Lev 6:13

Fire was to be kept constantly burning upon the altar without going out, not in order that the heavenly fire, which proceeded from Jehovah when Aaron and his sons first entered upon the service of the altar after their consecration, and consumed the burnt-offerings and peace-offerings, might never be extinguished (see at Lev 9:24); but that the burnt-offering might never go out, because this was the divinely appointed symbol and visible sign of the uninterrupted worship of Jehovah, which the covenant nation could never suspend either day or night, without being unfaithful to its calling. For the same reason other nations also kept perpetual fire burning upon the altars of their principal gods. (For proofs, see Rosenmüller and Knobel ad h. l.)