It's easier to block a staff that swing at you than to block a spear that stab into you. Since jab is like a spear stabbing, to use jab to set up your next move can be difficult.
The jab is a linear punch, it's not that easy to block it compare to a circular punch such as a back fist. If you want to use a jab to set up something else may be difficult since no arm contact is established. But if you use a back fist, since it's so easy to block it, 99% of the chance that your arm can contact on your opponent's arm. You can then change your punch into a pull and set up for your next move (such as to pull your opponent into your punch).
In other words, if you want to establish arm contact, the back fist or hook punch are better choice that the jab.
For example,
- You throw a back fist.
- Your opponent blocks it.
- You grab on his wrist, use another hand to control his elbow, and then ...
A punch can be a punch followed by a pull.