Unity 5 already comes with all native shaders installed, so, why have a separate folder to download named built-in shaders? Thanks!
The ones to download seperately are the shader sources. The uncompiled files with readable shader code as opposed to the already compiled ones that are included in the Editor by default.
It’s for if you want to modify the code.
Just like with any other Unity version - it’s the source code for the built-in shaders, if you want to learn how they are written, or want to copy them into your project and modify them in some way.