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?
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:
In halachah, we know what we may not do on Shabbos by looking at the activities that had to be done to build the Mishkan and its components.
The blueprint of the Mishkan in fact serves as the pattern for how to make ourselves and our world into a "dwelling-place for G-d in the lower realms." (In Chassidic writings this is associated with the previous point: our weekday activities, consisting of the same 39 categories of work done during the construction of the Mishkan, are indeed meant to turn this world into a Divine home on earth, of which the Mishkan was an example.)
Indeed, there is an idea (נפש החיים שער א פרק ד) that the Mishkan in fact represented, in its design and layout, the various parts of the human being's physical and spiritual makeup, and was also a microcosm of the universe.