How does w8 form effects revenue made on PPC/CPM/etc programs

Programs such as TheNewsRoom request to fill W8 in case the publisher is non-US citizen. How does this effect the earnings? Who is actually reducing the taxes? Is it USA before sending me the check with the payment, or is it my own government where I live? And how much is reduced?
I know it isn’t 100% related to Pay Per Click but this is the closest it gets to earnings revenue on these kind of programs.
I Appreciate the help.

Original post by icetsh