Why do I need to write shaders in unity3d?

You don't need to write any shaders if you are happy with the functionalities of the included shaders, community provided shaders or asset store paid shader packs.

If at some point you want to have a shader do a specific task, that is not available in all above shaders, you will have to write your own one. Custom lighting, like tanoshimi said, for example.

If you are a beginner, following some tutorials, making some games for the sake of learning it, you will not need any knowledge in writing shaders or how they work.

About 85% (I estimate) of games made with Unity ("Indie" games like slender clones included) don't use custom shaders except the unity-included shaders.

If you want to make a game that is based on custom visual effects and art, it could be time to look into it.