1. being of use or service; serving some purpose; advantageous, helpful, or of good effect: a useful member of society.
2. of practical use, as for doing work; producing material results; supplying common needs: the useful arts; useful work.
1. being essential, indispensable, or requisite: a necessary part of the motor.
2. happening or existing by necessity: a necessary change in our plans.
3. acting or proceeding from compulsion or necessity; not free; involuntary: a necessary agent.
a. (of a proposition) such that a denial of it involves a self-contradiction.
b. (of an inference or argument) such that its conclusion cannot be false if its supporting premises are true.
c. (of a condition) such that it must exist if a given event is to occur or a given thing is to exist. Compare sufficient ( def 2 ) .
An extremely important perspective to consider.