The Meta Pass decouples the albedo and emissive Enlighten uses from the regular shader passes.
The way an object looks is defined by its shader. Unity 3 and 4 used a simple mapping from material properties to lightmapper material properties. It worked somewhat, but was based on naming conventions, tags and strings. You couldn’t do any custom surface properties - it was basically hardcoded.
In Unity 5.x, albedo and emissive can be rendered using a special Meta shader pass. This is rendered in lightmap space, using the GPU. That means that how the object looks on screen and how it looks to the lightmapper is separated and you can customize the shaders. Also, you can control GI without affecting the shader used for realtime rendering.
This is how we handle albedo for metallic surfaces internally. Enlighten handles diffuse transport and uses surface albedo on each bounce. Metallic surfaces with black or almost black albedo would not bounce any light. The shader pass that renders albedo biases it towards a brighter color with the hue of the metal. Dielectric materials (wood, plastic, plastic, stone, concrete, leather, skin) have white specular reflectance. Metals have spectral specular reflectance.
Please note: Using a Meta Pass is not as fast as DynamicGI.SetEmissive but more flexible, because you are not limited to a single color.