What is the purpose of a ballast for aircraft?

Ballast in an aircraft is used to move the center of gravity (CG) to the required point. Weight and balance is used to keep safe operating parameters, and reduce fuel burn of an aircraft.

Safety: If and airplane slows and not enough air is moving over the wings it will stop creating enough lift to stay aloft. This is known as a stall. If the CG is correct the aircraft will want to "nose down" allowing the pilot to gain airspeed as the plane falls towards the ground. If the CG is to far aft it will be tail heavy. This could cause the aircraft to fall tail first towards the ground. I'm sure we could guess the potential outcome.

Fuel burn: To maintain the correct attitude in flight the aircraft is trimmed by moving the horizontal stabilizer. Anytime you trim an aircraft, or move a flight control, you are creating drag. To offset the drag the engines have to produce more thrust and that requires more fuel.

Having the CG correct, in range, allows for safe an efficient flight. In commercial aviation we prefer to load the freight and luggage in a manner that puts the CG in range as opposed to using ballast. Ballast increases weight and that increases fuel burn.