The grand jury has a limited term.
If people obstruct the investigation, say by lying to the grand jury, then the time will run out before the grand jury has gathered enough information to frame an indictment. Seeding confusing through obstruction of justice is one way to run out the clock on a grand jury's term. It means that the investigators have to spend (waste) time sorting out real events from planted fake ones. It's bad enough to have to sift through faulty memories without the additional impediment of deliberate lies.
That another grand jury is being empanelled to start over with what is now known says that there is still quite a bit still to investigate.