Why do I need to write shaders in unity3d?

I watched a tutorial which is Unite 2015 - Writing Shaders.
Yilmaz Kiymaz (Nordeus) explains the basics of shaders, how they work and how they’re written.

However, I don’t have any idea why I have to write shaders? What is point in writing shaders myself?
Why do I need to write shaders in unity3d?

I need clear clarification about it.

Shaders are the parts of your program that run on your graphics card and draw pixels to the screen. Without them, you wouldn’t have any graphics.

Unity comes with some built-in shaders and you can use them if you only want basic materials. As you start to develop custom lighting models or special effects, you’ll probably want to write your own however.