The ultimate goal and purpose of astronautics is to gain for man himself access to space and then to other worlds. The guided missile does not carry a man. It is a bridge between the space-flight concepts at the beginning and the space-flight reality yet to come.
-- General Dynamics/Astronautics, Centaur Primer: An Introduction to Hydrogen-Powered Space Flight (San Diego, 1962), pp. x-x1.
Missile's and drones are boring, you gotta stick a warm body in that thing to get any sizzle for your steak (rule 153)!
Seriously though, UAVs are great because they offer the opportunity for rapid, (relatively) inexpensive iterations of
concept --> design --> test --> field --> repeat
Probably the least appreciated, but most important aspect being the knowledge you can only earn by actual operating the hardware "in anger". Making sure that the guys who designed this one, will be able to benefit from insight gained from fielding to design the next one is pretty crucial. When you have decades long development programs, that loop just aint gonna close.