Вайикра 24:7 Комментарий: Rashi, Ibn Ezra, Rabbeinu Bahya & Chizkuni

ונתת על המערכת AND THOU SHALT PUT UPON THE ROW — i. e. upon each of the two rows [PURE FRANKINCENSE]; there were two bowls for this frankincense, each filled with a handful of it.

The frankincense that was with the bread became a fire-offering to God; the bread itself belonged to the kohen. This passage probably appears here because the community of Israel has a permanent obligation to offer the holiday burnt-offerings, as well as to supply oil and bread.

ונתת על המערכת לבונה זכה, “You shall put on (each) stack frankincense, etc.” The twelve loaves of bread mentioned here and the frankincense were both mandatory so that the absence of either invalidated the whole procedure (compare Menachot 27). They related to each other just like the etrog and the lulav, or the blue wool thread and the white threads on the tzitzit. (according to an opinion expressed in Menachot 38 that when תכלת, the blue colour needed for that thread is available, a garment which has only white tzitiyot is not fit to be worn).
The reason that the מנחה offering of a Sotah (woman suspected of marital infidelity) or the מנחה offering of any sinner was unaccompanied by such frankincense as is mentioned here as mandatory is that the Torah expressly forbade it (Leviticus 5,11 and Numbers 5,15) in order to express G’d’s disapproval of the persons having to bring these offerings, i.e. that at that time and until atonement has been obtained the attribute of Justice is poised to strike such a woman.

והיתה THAT IT — this frankincense — MAY BE

ללחם לאזכרה ON THE BREAD FOR A MEMORIAL — because of the bread itself nothing was offered to the Lord, but the frankincense was burnt when the former was removed from the golden table every Sabbath. It (the frankincense) thus served as a memorial for the bread, because through it, it (the bread) was recalled to memory Above (the bread itself was not offered) just as the “handful” of flour and of oil (Leviticus 2:2) was “the memorial portion” of the meal offering being the only part offered, while the remainder was eaten by the priests just as this bread was.