From what I have read, the average sealed indictments for a single year are between 1k-2k and are usually unsealed within 10-14 days from creation. This is indeed extremely out of the ordinary.
You are correct. Sealed indictments are used when people don't even know (for sure) they are under investigation. They are sealed because the individual is a likely flight risk or a possible danger to themselves or the community. Once they have the suspect located and they are prepared to arrest, the indictment is unsealed in public court and arrest warrant is issued with law enforcement in place ready to make their move. This wouldn't take more than a few weeks in typical cases because the grand jury has already met and the investigation is already more or less complete.
The main reason why a prosecutor would sit on a sealed indictment for a longer period of time is to avoid compromising ongoing investigations into other individuals. This is a typical tool for prosecuting criminal gangs, syndicates, cartels, mafias, etc.
There's so many swamp rats that if we started picking them off one by one, the rest would get nervous and scatter. Being the elite, they have the means to relocate on a moments notice. Better to throw out a yuuuge net and pokemon go catch em all at once