If some thing is hidden behind other from camera that are not rendered in unity this is occlusion. I think this is really good feature then why is it not implemented by default? Does any one having any idea or suggestion why I need to implement it manually? Any help would be appreciated greately…
Occlusion Culling is available in Unity but it is a Pro only feature.
Now why it is a pro only feature is a business tactic question. But lets hope maybe sometime in future it will be made available in Unity free as it happened with NavMesh in Unity.
Now as to why it is not used as a default standard in Unity itself, there is a very nice blog post over Unity blog about Occlusion Culling in Unity 4.3: Best Practices.
The excerpt of blog post is that not everything is ideal for occlusion culling so it is not always useful to have occlusion culling as a mandatory part of the engine unlike frustum culling and better left at developers discretion to wherever required.