There are a variety of reasons to seal an indictment (which will only remain sealed until the defendant is arrested), one of which os to facilitate arrest and avoid flight, another is to protect information in the indictment for as long as possible, especially if there is information that may be sensitive to ongoing investigations beside that of the person indicted, or if there is concern for the safety of victims/witnesses identified in the indictment.

> I guess this is to preserve the person that is accused

No, it is not. It is mostly to protect against actions of that person or their confederates, not to protect them.