As we can see in the next image, you can see what will be rendered by the main camera in the Scene view. Only a portion of the terrain is rendered, like if it was sliced on the needed angle, and other geometry of the level is well-hidden. In addition, you can visualize Visibility Lines that are the actual possible sight lines of the main camera at the current position and orientation:
Understanding how occlusion culling works is crucial to optimize a game that is supposed to hit the shelves. While it may seem easy to get it kicked, you may have to experiment and try different baking values before you find the optimum.
Ticking the Portals visualization option will also show calculated portals, areas that determine when a previously-hidden part of the level should be shown. You can see the difference between plain Frustum Culling and Occlusion Culling by selecting and deselecting the Occlusion culling option. In the preceding screenshot, you can see how the Frustum Culling removes the unnecessary parts of the level.
In the next similar image, with Occlusion culling enabled on the Camera component, you can finally see the result of the occlusion and how much geometry has been cut out compared to the previous image:
The results in terms of number of rendered triangles will be substantially different with Occlusion culling enabled on the camera.
In the next screenshot, we can observe from the Statistics overlay the number of draw calls (Batches) and the number of triangles and vertices rendered with Occlusion Culling enabled on the camera:
