When we look at the structure of parshiot Terumah and Tetzaveh, we can see that they have a logical order: first, the inside vessels (ארון, שולחן and מנורה), then the walls of the משכן itself, then the outside מזבח, then the courtyard, then the clothing of the כהנים. But somehow, the last of the inside vessels, the מזבח הקטרת, is left for the end:
All the commentators note this, and all offer reasons why it is singled out. The general answer is that it is somehow different; its function is not the same as the other vessels in the משכן. This is the Seforno:
The other vessels symbolized the bringing of the שכינה down to Earth; the מזבח הקטרת is to honor ה׳ for manifesting; it’s more for us than for ה׳. How the Seforno describes the symbolism of the manifestation is interesting:
The quote is from the story in מלכים after אלישע has taken over from אליהו:
He has a very anthropomorphic approach to the vessels of the משכן; while the ארון is often called the “seat” or “bed” of the שכינה (Rabbi Shulman has talked about the double symbolism there, with the כפורת as the throne (ה׳ יושב הכרובים) and the ארון proper as the “resting place”), seeing the שולחן and מנורה as part of the furniture of ה׳'s “apartment” is unique. But the idea that the מזבח הקטרת is different is brought up by other commentators. The אור החיים demonstrates this from תנ״ך, when שלמה builds the בית המקדש:
And the משך חכמה demonstrates this from the הלכה:
The idea is that the מזבח הקטורת wasn’t critical for the completion or the operation of the משכן; the important thing was the offering of the קטורת. The מזבח was just the place to put it. And what was the purpose of the קטורת? The Ramban gives an answer similar to Seforno, that it was to give כבוד to ה׳, but adds a dimension:
The קטורת is necessary to allow us to experience the presence of the שכינה. It is there to protect us from the overwhelming experience of ה׳'s transcendence—(שמות לג:כ) כי לא יראני האדם וחי. Getting it wrong is fatal:
But used appropriately it is life-saving:
But what is it about the קטרת that makes it so important? It is the smoke of the burning incense that evokes another cloud of smoke:
The ענן of the קטרת is a symbol of the ענן of the שכינה that would appear whenever ה׳ would manifest:
And it evokes the original ענן of עשן from הר סיני:
The ענן wasn’t the שכינה itself; that is insubstantial. It was the physical marker of the presence of the שכינה that served to shield mortal human beings from the problem of לא יראני האדם וחי. Even משה needed that protection.
משה had to wait until the ענן entered the אהל מועד; the unshielded כבוד ה׳ prevented him from entering.
So the placement of the מזבח הקטרת in our parasha is intentional. The משכן is the אהל מועד, the place of meeting between ה׳ and us. Parashat Terumah focuses on the bringing the presence of ה׳ down to Earth; parashat Tetzaveh focuses on the human role in serving that presence. The מזבח הקטרת introduces the tools that we need to make that connection possible, first by introducing a little distance with the ענן הקטרת, then continuing in the next parasha with the מחצית השקל “לכפר על נפשתיכם” and the כיור to purify the כהנים, and culminating in the שבת that serves as the אות “ביני ובין בני ישראל”, a ברית עולם of the our connection to הקב״ה.