Most indictments are never sealed (if the accused has been arrested on other charges, for example, there isn't much use in trying to keep it secret from the accused), and the ones that are always become unsealed in order to go to trial. There are a number of reasons to file an indictment under seal - ranging from juvenile records to managing flight risks of the accused.

The one that is probably most pertinent in this case has to do with testimony in front of a grand jury: both encouraging it to occur, and preventing it from being tampered with by the accused.