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Occlusion isn't really a shader like the ones listed above. It doesn't actually shade the surface itself, but gives a value from 0 to 1 that indicates how much of the backdrop is visible to the surface at any point. This value is then generally used to control other nodes, like specifying that recessed areas of the surface would be less diffuse or reflective. On its own, because it's a scalar node and doesn't support color vectors, it will only shade an object in greyscale, with white appearing on the surface where there's no other geometry between it and the backdrop (or maximum range), and black meaning that the backdrop cannot be seen at all from that point (or there's something blocking the surface within the range specified in the node). Because of the way a surface appears with this, it's frequently used to simulate radiosity. In fact, with Backdrop Radiosity and a white backdrop lighting a white, untextured object you would get virtually the same results as with this node. But where the shading of an object using Backdrop Radiosity is affected by the color of the backdrop, the shading using this node alone will always be greyscale, and it completely independent on lighting, light color or backdrop color. You'll get the same result with a black backdrop as with a white, or blur, or HDRI environment, etc.. Generally, you'll use this as a controller for another node, so its uses can be for anything really.