The secrecy of grand jury proceedings obscures the process and evidence, and can lend unearned legitimacy to prosecutorial decisions that protect the powerful from accountability and over-punish the marginalized.
The very threat of the secret proceeding is in itself terrifying to people. The secrecy of grand jury proceedings fuel paranoia and fear, running contrary to our ideals of open courts. When people learn about the secrecy of grand juries, they are often surprised that they are constitutional and compare them to the Court of Star Chamber, which existed in England from the 15th to 17th centuries.
Surprisingly, apart from the fact that the grand jury itself does not impose punishments, the biggest difference between the grand jury and the Court of the Star Chamber is that the Star Chamber proceedings were in fact largely open to the public.