For a minister of defense/secretary of defense type role you would hope they had been a career military person, specifically an officer. Officers, at least in the United States, are taught many things from tactics to military history, they understand command structure, they understand procedure and policy, as a career officer they would have years of experience in various commands serving in various capacities.

If you take John Smith from ABC Corp that got a degree from Harvard and is your golfing buddy and give him control over it, sure he's great at handling the civilians and the fobbits but he has no understanding of how a military operates. When generals/admirals tell him things he has no good context.