Having them sealed helps prevents the perpetrators (those people or corporations named in the indictment) from silencing witnesses, destroying evidence, fleeing the country etc. I don't think FOIA is applicable.
Unsealed means:
After the prosecutor studies the information from investigators and the information they gather from talking with the individuals involved, the prosecutor decides whether to present the case to the grand jury. When a person is indicted, they are given formal notice that it is believed that they committed a crime. The indictment contains the basic information that informs the person of the charges against them.
Having them sealed helps prevents the perpetrators (those people or corporations named in the indictment) from silencing witnesses, destroying evidence, fleeing the country etc. I don't think FOIA is applicable.
Unsealed means:
Sauce here: Justice.gov