Resistance bands were first used in many types of physical rehabilitation and sports medicine. When training in specific sports it is common to work specific muscles. Many of these exercises cannot be done with weight machines. When these muscles are strengthened helps to avoid injuries.
If you have been injured, bands can come to the rescue. When you do strenuous exercise with an injury, you there is always a chance of worsening your injury. When using resistance bands it makes it easier to isolate muscles without stressing out the injured area. When you are able to continue to exercise your uninjured muscles, the injured parts of your body benefit from the increased blood flow, there by helping speed up.