More power can be mustered with the strike if the hips are stabilized, the shoulders displaced forwards, and the feet grounded from their footwork. This is subject to the art and the user, but a jab can muster more of the body than simply the arm's extension and muscle.
A jab can also be a foot strike with the frontal foot in a sideways stance that "pokes" at the opponent with the intent of fending them off, or causing "stunning" pain to the shin, groin, abdomen, or higher targets. This "foot fencing" is designed to follow up with another technique or to buy time.
"I bashed him with a one-two."
"You dummy, the jab-cross! It was the cross that sent him to the dentist, heh!"