well a lot of people now have covered it.. but I will try to explain it in another way.
Earl was the first guy picked for a reason in the JS PC era.. He allows us to play the style defense we play.
In order for it to work, the corners need to be able to play press, the strong safety plays up right with the line backers just a bit behind them, and the free safety has the back half. What makes earl special is his ability to cover the left and right and back zones solo. All players in the scheme need a great football IQ, however the free safety needs Instinct as much as IQ. so.... you need the following in your free safety.
1) the ability to anticipate and react to the ball.
2) Incredible speed and burst movement.
3) Need to be able to tackle well.
all three of those are very rare in the nfl at safety, there are only a handful of guys that can do that. or that ever could to that level. so with that being said, he would be worth it most likely whatever it is we paid him.
Hopefully that explains it that's my short version.