Let's face it, Bill Clinton is not stupid, surely he knew that meeting Loretta Lynch privately on her government jet would cause controversy, and that it would not look good for Hillary, even if they only talked about grand kids. So why did he do it ?
The answer is to paint Loretta Lynch into a corner, so that the only option left to Lynch is taking the high road of following FBI's recommendation in the email investigation.
Why would he want Lynch to do that ? Because he knew, more than a week before the official announcement, that FBI director would not recommend indictment. And since Lynch had declared she would follow FBI's recommendation, it's virtually impossible for anyone to pressure her into prosecute Hillary on her own.