Why a machine
Hip thrust machine vs barbell and bench
A barbell hip thrust means a loaded bar across the hips, a bench that slides, and plates to roll into place for every set. A hip thrust machine fixes the back pad and the pivot, puts the load on a padded belt or lever instead of across the hip bones, and lets you set up and change weight in seconds. That is the whole case for one: the same movement, faster to load, with the pad and the platform where they should be every rep. For a studio or a shared floor it also means one station that a new user can figure out without coaching.
Hip abductor and adductor machines train the outer and inner thigh with a seated, pin-selected movement that free weights do not reproduce, which is why most strength floors keep one next to the hip thrust.