Why are many muscles at the effector end of the reflex arc found in antagonistic pairs?

Agonist and Antagonist muscles frequently happen in pairs, named antagonistic pairs. As one muscle pacts, the other eases. An illustration of an antagonistic pair is the triceps and biceps; to contract - the biceps contracts to boost the arm while the triceps relaxes. Therefore, antagonistic muscles have contrary actions. This is frequently needed for a reflex.