The best subwoofer box works by helping to reduce distortion in your sound system. By providing a place for your subwoofer to sit, it also gives a greater surface area for the soundwaves to ricochet off.
When this happens, the sound is projected back forward, toward the area you wish to subject to the music. Meanwhile, the air itself stays within the box and backward waves are held by the material. Thus, the music sounds louder, clearer and overall holds more depth.