Final answer:
A back-to-back bend is necessary when attaching a piece of conduit between outlet boxes on facing walls.
Step-by-step explanation:
A back-to-back bend is necessary when attaching a piece of conduit between outlet boxes on facing walls. This type of bend allows for the conduit to make a 180-degree turn, keeping it parallel to the walls. It is commonly used in electrical installations to route conduit between two parallel walls or between outlets.