Imagine modeling realistic clouds with geometric primitives. It would require a lot of computational power to actually create and place huge numbers of drops of water to create this effect. Volumetric rendering is an effective technique for modeling visual effects with huge numbers of fine particulates, like clouds.
To implement this technique, I built a camera, ray caster, and a voxel buffer. Then I filled up the voxel buffer with Perlin noise and simulated light being emitted, absorbed, and scattered by a large number of particles in the volume.