A writ of certiorari is almost always petitioned for during the course of a trial not yet completed, when a decision of the trial court is felt to potentially imminently result in manifest injustice. If the petition is granted, and the higher court issues a writ, then the lower court must often suspend proceedings until the issue is passed on by the higher court so that the trial can proceed in a fair manner and no prejudice is suffered by the moving party.
A petition for writ of certiorari will only be granted in cases where the higher court conceives that there is no other reasonable remedy at law, and both justice and judicial economy (as in obviating the need for a new trial) will be served.