...as the H-Back because it allows Garrett to disguise his plays.
Witten, Bennett and Phillips gives hims 3 good blockers who can block people in space or release into pass patterns.
With Anderson you have lead blocker on runs, blitz pick up and dump off routes.
With Phillips at "H" you can run him in motion engaging a linebacker, then either release him or crack down on a LB or D-Lineman.
He will lead block on runs, blitz pick up, dump off, motion to block, motion to seam route, motion to flat...he's just more versatile.
He's in there because Garrett must trust this guy to know the play packages and Tony's audibles and does his job.