What is the purpose of federal district courts?

The purpose of federal district courts is to handle small cases, such as those between businesses and people suing other people. The purpose of federal district courts is to handle the small problems that occur that need settling.

Another View: US District Courts handle ALL matters (large and small - civil and criminal) which involve federal law and which originate within their jurisdiction. They are the lowest level courts of original jurisdiction within the federal system.

In serious criminal cases, district courts convene panels of citizens, which are known as grand juries (to hear evidence of a possible crime and to recommend whether the evidence is sufficient to file criminal charges, there can be as many as 16 to 23 people, also they are not used in civil cases).