Both traditionally and in modern times, women will wear waist beads to get/keep their bodies intact. It is said that the beads shape your body and keep the waist small and hips accentuated. It was (and still is) used as a measurement tool.
The beads do not stretch, so if/when the waist beads start to feel a little tight, it's a sign that there was some weight gain, and vice versa.