The Senate has the authority to issue supeonas. Congress does NOT conduct criminal investigations. Hearings allow them, along with IGs to oversee the executive agencies. If suspected criminality is revealed by either then criminal referrals are sent to the DOJ. Hearings are also meant to inform the public so we can track what happens. If the DOJ/FBI are corrupt and drop the ball as they did with Hillary then its our job to vote in a different head of the executive branch/POTUS. Thanks to Congressional hearings voters did just that by electing Trump and rejecting Hillary.