Jerry is hiring Turner as HC because he cannot contractually pursue Norv the Offensive Coordinator.
Jerry has hired the man he wants as head coach (Garrett), but due to the Turner issue, he held off of giving him a title until they got feelers in on what San Fran might do. Word back was that San Fran would raise Norv to keep him from interviewing, period, so Jerry then decided he wanted Garrett to get some "seasoning" for a year or so. After the season is over, Norv and Garrett will switch positions and Norv will become the highest paid offensive coordinator in the NFL.
What the media is discussing as a possible "Lose-lose" situation for Turner becomes a "win-win". He will either become a viable HC candidate again or he will become the highest paid OC in the NFL. Garrett agrees because he will be HC and have his friend Turner running the offense. Win for Jerry, because he has found a way to circumvent the system and get what he wants.