When you punch or kick, the limb should extend but does not lock out. Watch people who fight for a living, such as boxers or MMA fighters, they do not lock out their elbows when punching. Not all punches make contact with the intended target. Hitting the wrong target, one that does not give way, may injury a straight arm. A miss, where arm the locks out as fist suddenly stops forward motion, also may injury the elbow.
Have you ever been walking down a city street, stepping off curbs as you look at the sights, and stepped off a curb that was just a little higher than normal? Your stepping leg extends a little more and the knee locks just as the foot hits the ground with a thud that sends shock waves throughout your body and even jolts the brain a little. When the elbow or knee locks out, there is no way for the body to absorb the shock of a strike. Instead of the ligaments, muscles, etc. absorbing the shock gradually, the joints get slammed together and the shock reverberates throughout the entire body.
When you fully extend the arm or leg in an attack, it is easy for the knee or elbow to move a little more than it may safely handle and it may hyperextend, causing an immediate major injury or cumulative minor injuries that will manifest later as a major injury.