Why should ultra sound therapy be used for broken bones?

It speeds up healing. All day long our bones undergo a process called remodeling, which keep the bones healthy by fixing small breaks, all the time. Remodeling, bone healing, is slower when there's no stress on the bone when it is immobilized: when you can't walk on it, or when it's in a cast and you can't move it. Ultrasound acts like stress on a bone and tricks the bone into growing.
Assist bone healing. Normally bones have the ability to repair and regenerate themselves. Sometimes, delayed union (bones taking longer to heal) or nonunion (bones not healing at all) can occur. In these instances, a bone stimulators can help improve the biologic conditions for he bone to heal. Ultrasound therapy has been shown to stimulate some of the pathways involved in fracture healing.