Unlike other individual rights guaranteed by the Bill of Rights, the right to a jury trial is also guaranteed in the body of the Constitution. The Sixth and Seventh amendments were added, however, to make sure such trials were fair and not subject to manipulation by the government. The Sixth Amendment deals with criminal prosecutions; the Seventh, with civil cases.
The Sixth goes beyond the Constitution's bare guarantee of trial by jury to insist that the trial be "speedy" and "public," that the jurors be "impartial," that the accused be informed of "the nature and cause" of all charges, be able to compel testimony, examine the evidence and have a lawyer.
The evils the Framers sought to avoid were lengthy incarceration based on unproven charges, and secret, rigged or show trials, which were not uncommon in English jurisprudence. The Seventh's intent was to make sure ordinary citizens the jury rather than officers of the state the judge had the right to decide what had happened in civil disputes. It says that "no fact tried b a jury shall be otherwise re-examined in any court, than according to the rules of the common law."