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.