A district court is a court that presides over a certain region. District courts can be federal or state. Each type of court has different types of cases that it tries. For example, a state district court may try an assault case that occurred within its jurisdiction. District attorneys lead the district courts.
In order for all the cases of a region to be heard, there must be enough courts to hear them. So, in order to handle the justice system requirements, there are district courts. These courts have a certain jurisdiction and they process the cases for this jurisdiction. If there were no district courts, then the legal system would not be able to fulfill its duty of trying cases. There would simply be too many. This is why district courts are necessary.