First off, it's hard to play good cop, bad cop to get someone to change their mind if someone isn't willing to play the bad cop.
Second, my boss has been at the company a number of years, and so can tell me what my sponsor is going to be looking for, and any political landmines I might run into.
Lastly, my boss is the one who goes toe to toe with upper management to get the raise that he thinks I deserve, not just the default raise.
And I don't think anyone would question the usefulness of "general politics person" and "general team sanity person". The problem is, the roles are convoluted with "the ruler," and the question is, is "the ruler" really necessary, and even if the ruler is necessary, is this really the same person as the "sanity" and "politics" people.