T'rumah: make for Me a sanctuary
A common answer is that God doesn't need the mishkan; the people do. This is a people living in a world of physical gods and their accoutrements, and they need something physical with which to interact with God. We are not so different today; we still build grandiose places of worship that we think worthy of a transcendant God. Perhaps we need that (though plenty of basement and camp minyanim testify otherwise), or perhaps we think that we owe God our best when it comes to physical spaces.
I think there are two reasons that the torah specifies the mishkan in such detail. First, it takes the decision away from the people: instead of arguing endlessly over the design, or fretting that they aren't doing enough, the people are told "build this exactly". Would that our own building committees had such clear direction. :-)
The other reason, I think, is to limit the scope of the effort. Left to our own devices, many of us will keep tinkering with something in pursuit of perfection, which never comes. If we'll do it for our work, our crafts, and our hobbies, how much moreso would we do it for God? And if we, with the benefit of 3300 years of history since Sinai, will do it with God, how much moreso would the founders of the religion fret over the details?
The danger of this is that it is too easy to end up so focused on the task at hand that you miss the larger picture. Judaism isn't about the physical space, then or now -- it's about relating to God and each other. Without the limits of a closed design, would the people have been able to move on from planning and building to actually worshipping and serving God -- the purpose for which the mishkan was built? When we get wrapped up in the details of our worship spaces, or any of the other issues that go into bringing a community together to serve God, do we risk missing the larger point?
My regular Shabbat minyan meets in a basement. People dress casually, and there are no elevated places in the room, or microphones, or other barriers between the leaders and the other congregants. I appreciate the beauty of our sanctuary and our chapel, but it is on Shabbat mornings in the basement that I most feel a part of a community that reaches out to God.
another reason
Re: another reason