Why does the Torah feel the Need to tell me the rules for making the Mishkan? First, it is no longer relevant, as, even if we build, we will build a Beis Hamikdash. Plus, the Medrashim say it will be pre-made anyway?
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Whether the third Beis Hamikdash will be pre-built is in fact the subject of a dispute among the Rishonim. Rashi (Sukkah 41a, ד"ה אי נמי) says that it will come down from Heaven ready-made; on the other hand, Rambam (Hil. Melachim 11:4) lists the building of the Beis Hamikdash as one of the things that Moshiach will accomplish (in fact, that he needs to do in order to be considered the definite Moshiach). There is also a famous reconciliation of these two opinions (I believe I've seen it attributed to R' Yehoshua Leib Diskin, the Brisker Rav) is that the building as a whole will come down from Heaven, but it will be up to us to put in the gates.
In any case, though, even Rashi might agree that the vessels and utensils of the third Beis Hamikdash have to be made by human hands. In his commentary to Ex. 25:9 (ד"ה וכן תעשו) he cites a couple of places where the Gemara explains that the instructions given in the Torah are indeed for all time: whenever one of the vessels needs to be replaced (or additional ones need to be made), they are to follow the patterns given here. So much of the description has practical value for that reason alone.
Aside from all that, though, there is a great deal we learn from the details of the Mishkan's construction: