"A machine can decrease either the input force or the input distance needed to do a given amount of work" statement best describes why a machine is useful.
Step-by-step explanation:
A machine is any kind of instrument, apparatus or mechanism which has the ability to reduce the amount of work done by humans or by mechanical force. It uses power in terms of electrical energy to generate the force acting on any object for a particular job.
So the work done is dependent on the force acting on any object and the displacement done on the object. As the power in form of electrical energy generates the force acting on the object, the machine can reduce the input force or input distance needed to do a given amount of work by varying the angle of acting of force.