His purpose is to lead and manage a team to win the Super Bowl. That's every HC's purpose. This idea that any HC must call plays is ludicrous since the league has been predominantly having coordinators call plays for years.
The issue I see with Garrett is that we clearly have a problem with how this offense plays. Even if it plays well, it can't play well consistently. I used to think that Wade was the real problem given how he ran practices. Now I see that Wade was only part of the issue as the offense is far more disciplined with out him. But we still have issue with an offense that lacks creative ways to get their best players the ball in open space, that requires too much clear domination of one-on-one battles, that has a faulty risk-reward principles and refuses to get on their QB when he audibles out of a running play again.
If Garrett can't fix it by now as the head coach of the team, I just highly doubt it will be ever fixed as long as he's the HC. I could be wrong, but at the very least I have my doubts regardless of who the O-Coordinator is.
YR