This practice is referred to as a "Change of Venue". A change of venue is issued when the court believes that the defendant cannot receive a fair trial within the jurisdiction. Often, this occurs when an infamous crime is committed in a small town. In such situations, rumors get around very quickly, so by the time the court is picking out a jury, the entire populace has a biased, or corrupted, opinion. Assuming the change is accepted, the trial will be held in a new jurisdiction, where the citizens (and jury) were not exposed to such rumors and biased opinions. The idea is to secure the defendant's right to a fair, impartial trial, as guaranteed by the United States Constitution.