God wanted a home. Not in the highest heavens where angels sing without ceasing. Not in the dazzling worlds of pure spirit. God wanted a home in the lowest, darkest, most difficult place in all of existence: right here.
Rabbi Schneur Zalman of Liadi builds this teaching on a famous statement from Midrash Tanchuma (Nasso 16): the purpose of creation is that God desired a dirah b'tachtonim (דירה בתחתונים), a dwelling place in the lower worlds. But the Tanya immediately challenges this. God fills all worlds equally. For God, there is no "upper" or "lower." So what does this mean?
The answer lies in the concept of hishtalshelut (השתלשלות), the cascading descent of divine light through ever-thicker veils of concealment. Each world receives God's life-force through more layers of cosmic "garments" until you reach this physical world, the very bottom, a place of such doubled and redoubled darkness that evil can actually declare, "I am, and there is nothing else besides me" (Isaiah 47:8).
And this is exactly the point. The higher worlds did not need the descent. For them, it was a step down from God's light. The entire chain of worlds exists for this world. Because when darkness itself is transformed into light, when the place that seemed furthest from God reveals His presence, the resulting illumination is infinitely greater than anything the higher worlds experience. Light that emerges from darkness blazes with an intensity that light which was never challenged simply cannot match.
This is why God gave the Torah to physical human beings in a material world. The Torah is the tool that lets finite creatures transform darkness into light, making this lowest world into the place where God's infinite light shines brightest of all.