There are numerous reasons a surgeon may use spreader grafts during rhinoplasty. The most common is to support the cartilages which form the middle part of the bridge after a hump is lowered. They can also be used to improve nasal airflow constriction, straighten a crooked nose, or to repair collapse after trauma. It is not necessarily the case that spreader grafts have to be used after osteotomies, although there may be other reasons they would be a good idea in any given surgery. It really depends on the nose, the anatomical issues needing to be fixed, and the goals of the patient. The video link shows an explanation of one use of spreader grafts for nasal valve problems.