You shall place the golden altar for ketores before the ark of testimony.
Meshech Chochmah: The pasuk certainly does not tell us to place the golden altar immediately in front of the aron, either within the Holy of Holies, nor right in front of it. We know where this mizbeach stood; it was displaced a considerable distance from the aron. It was further removed, in fact, than the shulchan and the menorah, both of which stood closer to the kodesh ha-kodashim. The simple reading of our text is an instruction that wherever it is placed, it should line up directly with the aron inside the kodash ha-kodashim, and not be displaced neither to the left nor the right.
Such a reading, though, is unsatisfying. If this were the Torah’s intention, the instruction belongs elsewhere. It would seem more appropriate in the section describing how Moshe set up the mishkan, and where he placed the kelim. A good candidate would be the pasuk that describes how Moshe placed the golden altar “in front of the paroches.” Similarly, the Torah even earlier describes this altar as standing in front of the paroches. At either one of these places the Torah could have underscored that the altar should line up in a straight line (along the front-to-back axis of the mishkan) with the aron that stood behind the paroches.
We could imagine a different purpose for our pasuk: informing us about the function of the ketores itself. There were those – notably the Rambam – that the aromatic ketores was meant to displace the otherwise foul odors that would seep into the structure of the mikdosh. We know the stench associated with abattoirs; the mikdosh was a place in which not only were animals slaughtered and butchered, but their fats were then burned day and night. The lingering effects would naturally be overpowering. The Torah, according to these sources, instructed as to burn powerful but sweet-smelling incense twice daily to counteract the less desirable smells.
Others strongly objected to this approach. If the function of the ketores were simply instrumental, why would the Torah list the ingredients of the ketores with great specificity, and forbid any change in the recipe, as well as using the special blend for any other purpose? Ketores figured in the avodah of the week of the mishkan’s inauguration, before many animals had been slaughtered, and when the structure was taken apart and reassembled each day.
We must conclude that the purpose of the ketores was not for the practical benefit of those who would come to the mishkan. Rather, it was what Chazal call tzorech Govoha/ a Divine need. This means that it was a necessary component in the precise manner in which Hashem wishes to be served in the mikdosh. Nothing in the avodah is arbitrary; the precise formulation of its requirements flows from its source in esoteric mysteries. Those privileged to have penetrated some of those mysteries are well aware of the lofty messages are incorporated in the ketores.
This, then, is the Torah’s intention in our pasuk. It describes the avodah of the ketores as “before the ark of testimony.” We are meant to understand that its purpose is not to serve any practical human need, but to serve the Divine Presence that rests upon the ark of testimony.
 Based on Meshech Chochmah, Shemos 40:5
 Shemos 40:26
 Shemos 30:6
 Moreh Nevuchim 3:45