necessary evil

necessity (quality or state of being necessary, unavoidable, or absolutely requisite)

unnecessary

needlessness

(traditional) customs or rules

a machine, a device; a contraption / a loom / an opportunity, a chance to do something; the right time or moment / something of great importance: the linchpin of the matter

indicates aircraft / indicates paper aeroplanes / indicates lives in video games

a trap or snare that works by beheading the bird or beast caught in it

manipulation (as in a device) / manipulation (as in a person): deception, taking someone in / getting on someone's right side; humoring someone; an effort to please someone

indicates machines / indicates airplanes