Jabs help find your range, distract opponents and set up the back hand. Jabs set up the back hand by acting as a range finder and making your opponent react or move. Once your opponent has reacted you throw the back hand because now you know where your opponent was planning to move to.
A quick, strong jab can tire and dismantle your opponent. Therefore the double jab is an important skill in boxing.
The double jab is often performed incorrectly even by competitive boxers that have had a few fights. With the double jab you take a small step with the first punch (jab), and another step with the second jab. These are small steps, not large steps.
In between jabs you need to retract your hand back to your face to your guard and get ready to jab again. So for example after you step and throw your first jab, bring your hand back to your face and have it in guard before you step again to throw another jab. By punching from your face you are also able to generate a faster, more powerful jab than if you were to leave your hand hanging around after the first jab.
Take a small step per jab, retract your hand back to your face to guard it and then jab from the face again. Be sure to begin the combination from out of range of the bag as if it were an opponent.