QUESTION: The first Mishnah in Midos begins by stating that there were three places in the Beis ha'Mikdash where the Kohanim stood guard (as the Mishnah in the beginning of Tamid also teaches).
Why does Maseches Midos, which deals primarily with the measurements of the various parts of the Beis ha'Mikdash, begin by describing the Mitzvah of standing guard in the Beis ha'Mikdash? This Mitzvah is not related to the subject matter of Maseches Midos. Rather, it is part of the daily routine in the Beis ha'Mikdash, and is recorded accordingly in Maseches Tamid!
ANSWER: The TIFERES YISRAEL (Boaz #1) cites the RAMBAM (also quoted by the TOSFOS YOM TOV in his introduction to Midos) who writes that the purpose of learning and knowing the measurements and plan of the building of the Beis ha'Mikdash is so that when the time comes for the Beis ha'Mikdash to be rebuilt, we will we be "Shomer" -- preserve and guard its plan and all of its details. The Beis ha'Mikdash is built according to Ru'ach ha'Kodesh, as the verse says in its description of the measurements with which Shlomo ha'Melech built the first Beis ha'Mikdash, "All of this is in writing from the hand of Hash-m, Who gave me understanding to know" (Divrei ha'Yamim I 28:19). The Tiferes Yisrael writes that support for the Rambam's words may be found in Yechezkel (43:11), where the verse says that the Jews must "preserve (Shomer) the form and the laws of Hash-m's house and build it."
However, the approach of the Rambam does not seem consistent with the statement of RASHI and TOSFOS, who write in a number of places that the third Beis ha'Mikdash will be built directly by the hand of Hash-m. Rashi in Sukah (41a, DH Iy Nami) writes that "the future Beis ha'Mikdash, for which we are waiting, will be revealed and descend from heaven, as is said, 'The sanctuary, Hash-m, which Your hands have established" (Shemos 15:17). (See also Rashi to Rosh Hashanah, end of 30a; Tosfos to Sukah 41a, end of DH Iy Nami; Tosfos to Shevuos 15b, DH Ein, in the name of the Midrash Tanchuma. See also Bava Kama 60b, where the Gemara relates that Hash-m said, "In the future, I will build the Beis ha'Mikdash with fire," as is written, "I will be a wall of fire around Yerushalayim, said Hash-m" (Zecharyah 2:9). Similarly, in the Tefilah of "Nachem" recited in the Shemoneh Esreh on Tish'ah b'Av, we say, "... for You, Hash-m, burned it with fire, and with fire You will build it in the future.")
To explain why the first Mishnah in Midos discusses a Mitzvah that is not related to the measurements and plan of the Beis ha'Mikdash, the Tiferes Yisrael writes that Rashi and Tosfos mean that Hash-m will help us, through miraculous means, to build the Beis ha'Mikdash. Since it will be built by man (with Divine help), it will be necessary to know the Halachos.
(RAV YOSEF SHALOM ELYASHIV shlit'a, in his notes to Sukah (41a, p. 191), cites an additional explanation in the name of RAV YEHOSHUA LEIB DISKIN zt'l, which may cast further light on the view of Rashi and Tosfos. In the Musaf prayer of Yom Tov we request of Hash-m, "Show us the rebuilding of the Beis ha'Mikdash, and make us happy by its establishment (v'Samcheinu b'Tikuno)." The verse in Eichah (2:9) says that the "gates of the Beis ha'Mikdash sank into the mud" when it was destroyed, and the Midrash relates that the gates sank into the ground directly beneath where they were standing. The Midrash also says that in the time of the future redemption, the Jewish people will extract the gates from the ground and set them up in their correct place. Since the Gemara in Bava Basra (53b) rules that one does not acquire a piece of land until he performs an act showing his ownership over it, such as by setting doors in place, the setting up of the gates in their place will represent the completion of the building, even though the main part of the building will descend completely from Shamayim. Therefore, we say in the Musaf Shemoneh Esreh, "Show us the rebuilding of the Beis ha'Mikdash" -- a reference to the main building that will descend from Shamayim, and we also ask, "and make us happy by its establishment (v'Samcheinu b'Tikuno)" -- a reference to setting up the gates, which will be done by the Jewish people. Accordingly, since the Jewish people will have a part in the rebuilding of the Beis ha'Mikdash, it is necessary to learn, know, and be "Shomer" the Halachos.)
The Tiferes Yisrael adds that although the Beis ha'Mikdash will descend from Shamayim, we need to preserve and guard its form in our minds so that when it is built we will see that it was miraculously built exactly how the prophet foretold thousands of years ago. People will then recognize that it truly was the Ru'ach of Hash-m speaking through the prophet, Yechezkel, and that our ancestors were wrong by not listening to him. By seeing that Yechezkel's words were all true and accurate, the people will be ashamed of their sins and repent (see Yechezkel 43:10).
The RAMBAM (Hilchos Melachim 11:1), however, writes that the Melech ha'Mashi'ach will build the Beis ha'Mikdash. According to his view, it is easier to understand the Rambam's own reasoning that the purpose of learning Maseches Midos is to know how to build the Beis ha'Mikdash.
This explains why the Mishnah begins by telling us how the Beis ha'Mikdash was guarded. The purpose of learning Maseches Midos is in order that we should guard and preserve in our hearts the way it will be built.
The Tiferes Yisrael adds that this is the meaning of the verse, "I have set watchmen upon your walls, Yerushalayim, who shall never be quiet, all day and all night" (Yeshayah 62:6). "All day" refers to the guardians of the Beis ha'Mikdash when it stood. "All night" refers to those who "guard" it by learning its Halachos during the night of exile, when the Beis ha'Mikdash lies in ruins due to our sins. We must never be quiet, so that we will merit the rebuilding of the Beis ha'Mikdash, speedily in our days. (D. BLOOM)