They also do it to stabilize the area when they don't know what is wrong. Having it flop around can tear things or displace fractures that they don't know the existence or the extent of. Aircast it to be safe, get your xrays to evaluate for broken bones, get your MRI for soft tissue (ligaments, tendons, muscular issues) them diagnose and evaluate a plan going forward. There is no sense in NOT stabilizing it immediately.