As I have always understood the reasoning, there is a recognition that some cases involve enough public interest and emotion, that it may be difficult to find a totally unbiased jury.
(Note the American trial of O.J. Simpson, where at least 2 jurors admitted later that they intended from the beginning to find him "not guilty", despite overwhelming evidence.)

I suppose Michael, that what is going on is a system of checks & balances to allow the coroner's 'expert' opinion to counter poor understanding or prejudice in the jury.