Why does an action that requires quick movement use a reflex arc?

A reflex arc is a neural pathway that controls a reflex. In vertebrates, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain.

It uses a reflex arc because a reflex controls the brain and makes you think quick to do a fast movement that is known as a reflex, a example is something falling and your instinct is to catch it.

sitting in a chair getting hit with a reflex hammer in the knee