Fitzgerald did a pretty good job of explaining the nature of an obstruction charge, and why he might wind up indicting someone for obstructing an investigation that never resulted in an substantive charge. I can’t quote it precisely, but to paraphrase:
If a pitcher hits a batter in the head, the umpire needs to know whether it was deliberate or not. If someone throws sand in the umpire’s face to keep him from seeing the play, he might never know whether the beaning was deliberate. Throwing sand in the umpire’s eyes would still be a very bad thing to do.