It is especially important to be able to recognize the signs of injuries if/when the casualty is unconscious.
Why ? ... A person who is unconscious cannot say anything to explain to a first aider (or paramedic, or anyone else) what has happened, what hurts and where, and what he or she thinks the problem may be.
In the case of an unconscious casualty the best information available might be the physical signs of injury. However, a responsible person attending an accident or injury would normally also ask if anyone else saw or heard what happened and try to find out about both the incident and the general health (including any known medical conditions) of the injured person.