Why do you need the source code to write a utility when all you need is an API? I've written software for Linux and Solaris but never had to look at the source code, so I imagine it's the same for Windows.
Where in the comment is this statement?
Personally the primary reasons I would want the source code for the kernel and utilities would be 1. to assess its quality and, assuming the quality meets my standards, 2. to modify it to meet my own ends. In my case, the less I have to write things from scratch the better.
Let me know if you still have questions.
> Or I can write my own simple utility. I am given the full UNIX source code. Where is the source code for Windows?
I agree with your reasons, it is useful to have the source code. Just saying tat in the many, many years I've been writing software that's deployed on Linux, I've never had to look at it.