Exodus 30:8

and when Aaron sets up the lamps at twilight , he shall burn it, a regular incense offering before the Lord throughout your generations .

He must burn incense again when he lights the lamps at twilight so incense will burn regularly before the Lord for the generations to come.

And when Aaron lighteth the lamps at even, he shall burn incense upon it, a perpetual incense before the Lord throughout your generations.