Back facials were created in response to skin issues such as back acne and dry skin. Estheticians correctly implied that the face and back are not very different, and problems of the back were treatable with skincare tools and products already available.
The products used in a back facial are almost identical to those used in a regular facial but generally formulated slightly stronger, compensating for the fact that it must work through the skin of the back, which is thicker than facial skin.