They allow you to remove pilots from areas of danger. Since the machines do not house a pilot, there is no need for a cockpit and other accessories that a pilot requires in order to fly an aircraft. This cuts significantly on the costs required to build an aircraft.
Cost is a major consideration when it comes to building aircraft and other defense equipment and machines as there is usually not enough budget to fulfill all requirements. Military drones are being used for a number of purposes, most importantly intelligence, surveillance and reconnaissance (ISR).